A
- a-axisCrystallography
- One of the crystallographic reference axes used to describe a unit cell and crystal faces.
- Accessory mineralPetrology
- A mineral present in a rock in small amounts and not essential to the rock’s basic classification. Zircon, apatite, and magnetite are common examples.
- AcicularHabit & form
- Needle-like, with crystals that are very slender relative to their length. Rutile and natrolite can be acicular.
- Acid cleanedSpecimen careCollector
- Cleaned using an acid chosen to dissolve stains, coatings, matrix, or unwanted minerals.
- Acid testIdentification
- A chemical test using dilute acid to check for reaction, especially effervescence in carbonate minerals.
- Adamantine lusterLuster
- An exceptionally brilliant, diamond-like nonmetallic luster associated with high refractive index.
- AditMining
- A nearly horizontal passage driven into a hillside to access an underground mine.
- AdularescenceOptical phenomena
- A soft floating bluish or whitish sheen seen especially in moonstone.
- Aesthetic specimenCollectingCollector
- A collector term for a specimen valued especially for visual composition, crystal placement, color, contrast, and balance.
- AfterglowLuminescenceCollector
- An informal term for light that remains after the excitation source is removed, usually referring to phosphorescence.
Also: Persistent luminescence
- AgateCommon minerals
- A banded variety of chalcedony, commonly formed by successive silica deposition in cavities.
- AggregateHabit & form
- A group or mass of mineral grains or crystals occurring together. Aggregates may be granular, fibrous, botryoidal, radiating, or take many other forms.
- Air abrasiveSpecimen careCollector
- A specimen-preparation method using a controlled stream of fine abrasive powder to remove matrix or coatings.
- AllochromaticColor & optics
- Describing a mineral whose color is caused mainly by trace impurities, defects, or other non-essential factors.
- AlterationAlteration
- A change in mineral composition or texture caused by fluids, weathering, heat, or chemical reaction after the original material formed.
- AmethystCommon mineralsVarietal
- A purple variety of quartz, typically colored by iron-related defects activated by natural irradiation.
- AmetrineCommon mineralsVarietal
- A gem variety of quartz showing both amethyst-purple and citrine-yellow zones in the same crystal.
- AmorphousCrystallography
- Lacking long-range periodic atomic order. Natural glass is amorphous, although it may contain tiny crystals or inclusions.
- AmygdaloidalPetrology
- Describing volcanic rock containing mineral-filled vesicles called amygdules.
- AmygdulePetrology
- A former gas cavity in volcanic rock that has been filled with secondary minerals such as calcite, quartz, or zeolites.
- AnhedralHabit & form
- Lacking well-developed external crystal faces, usually because growth was constrained by neighboring grains.
Also: Xenomorphic
- AnionMineral chemistry
- A negatively charged ion or ion group. Oxygen, sulfide, carbonate, sulfate, and phosphate are important mineral anions.
- AnisotropicOptical properties
- Having optical properties that vary with direction. Most non-cubic crystals are anisotropic.
- ApatiteCommon minerals
- A group of calcium phosphate minerals; apatite is the reference mineral for Mohs hardness 5.
- AplitePetrology
- A fine-grained, light-colored igneous rock commonly occurring in veins or dikes and often associated with granite and pegmatite.
- Approved mineralClassification
- A mineral species whose name and status have been accepted through formal International Mineralogical Association review.
- AragoniteCommon minerals
- A calcium carbonate mineral, CaCO3, with orthorhombic structure. It has the same composition as calcite but a different structure.
- ArborescentHabit & form
- Branching in a more three-dimensional tree-like form. Native copper can produce striking arborescent specimens.
- Argillic alterationAlteration
- Hydrothermal alteration characterized by formation of clay minerals.
- ArsenateMineral classes
- A mineral containing arsenate groups, commonly AsO4. Adamite and mimetite are examples.
- Artichoke quartzHabit & formCollector
- A collector term for quartz showing strongly stepped, overlapping growth layers around the crystal.
- AsbestiformSafety
- A fibrous habit in which crystals grow as long, thin, flexible, separable fibers with properties characteristic of asbestos.
- AsbestosSafety
- A commercial and regulatory term for certain naturally occurring silicate minerals when they occur in specific asbestiform habits.
- AssociationGeologic occurrence
- The minerals commonly found together in a particular deposit, rock, or geologic environment.
- AsterismOptical phenomena
- A star-like pattern of light produced by reflection from oriented inclusions or structures.
- AventurescenceOptical phenomena
- A glittering effect caused by light reflecting from numerous oriented plate-like inclusions.
- AzuriteCommon minerals
- A deep blue copper carbonate hydroxide mineral, Cu3(CO3)2(OH)2, commonly associated with malachite.
B
- b-axisCrystallography
- One of the crystallographic reference axes used to describe a unit cell and orientation.
- BariteCommon minerals
- Barium sulfate, BaSO4, a high-specific-gravity mineral that commonly forms tabular or bladed crystals.
- Basal cleavagePhysical properties
- Cleavage parallel to the base of a crystal, commonly producing sheets or plates. Micas are classic examples.
- BasaltPetrology
- A dark, fine-grained volcanic rock rich in pyroxene and plagioclase.
- BeadLapidary
- A shaped piece of mineral, gem, glass, or other material pierced for stringing.
- Becke lineOptical mineralogy
- A bright line near a grain boundary used in microscope work to compare refractive indices.
- BenchMining
- A step-like working level in an open-pit mine or quarry.
- BerylCommon minerals
- Beryllium aluminum silicate, Be3Al2Si6O18, whose gem varieties include emerald, aquamarine, and morganite.
- BiaxialOptical properties
- Describing an anisotropic crystal with two optic axes. Orthorhombic, monoclinic, and triclinic crystals are biaxial.
- BirefringenceOptical properties
- The difference between refractive indices for light traveling in different permitted directions in an anisotropic material.
- BladedHabit & form
- Flattened and elongated like the blade of a knife. Barite and kyanite commonly show bladed habits.
- BorateMineral classes
- A mineral containing boron-oxygen groups as essential structural components.
- BorniteCommon minerals
- A copper iron sulfide, Cu5FeS4, that commonly tarnishes to purple, blue, and iridescent colors.
- BotryoidalHabit & form
- Composed of rounded, grape-like surfaces formed by closely packed globular growths. Hematite, malachite, and goethite commonly occur this way.
- BoxworkGeologic occurrence
- A network of resistant mineral ridges left after softer material between them has weathered or dissolved away.
- BrecciaPetrology
- A rock composed of angular fragments cemented or supported by finer material.
- BrecciationPetrology
- The process of breaking rock into angular fragments.
- BrillianceGemology
- The brightness created by light returned to the viewer from a gem or polished surface.
- Brilliant cutLapidary
- A faceting style designed to maximize light return and brilliance.
- BrittlePhysical properties
- A tenacity in which a mineral breaks, powders, or crumbles rather than bending permanently.
- BruiseCollectingCollector
- Fine internal fracturing or crushing beneath a specimen surface, often appearing whitish or frosted.
C
- c-axisCrystallography
- A crystallographic reference axis, often the unique or vertical axis in drawings.
- Cabinet specimenCollectingCollector
- A relatively large display specimen intended for a cabinet or showcase.
- CabochonLapidary
- A gemstone cut with a smooth rounded or domed top rather than flat facets.
Also: Cab
- Cactus quartzHabit & formTrade
- A trade and collector term for quartz whose main crystals are coated by a second generation of smaller quartz crystals.
Also: Spirit quartz, Porcupine quartz
- CalciteCommon minerals
- Calcium carbonate, CaCO3, known for rhombohedral cleavage, double refraction in clear crystals, and reaction with dilute acid.
- Calibrated stoneLapidaryTrade
- A gemstone cut to standardized dimensions so it fits commercially available jewelry settings.
- CameoLapidary
- A carved relief in which a raised design contrasts with a differently colored background layer.
- CapillaryHabit & form
- Forming extremely slender hairlike crystals, often finer than ordinary acicular crystals.
- CaratGemology
- A unit of mass used for gemstones, equal to 0.2 gram. It should not be confused with karat.
- CarbonateMineral classes
- A mineral containing carbonate, CO3, as an essential component. Calcite, dolomite, and malachite are examples.
- CarbonationAlteration
- Alteration involving addition of carbon dioxide and formation of carbonate minerals.
- CarvingLapidary
- Shaping gem or ornamental material into a figure, relief, vessel, bead, or decorative object.
- CastAlteration
- A mineral replica that fills or coats a cavity left by a pre-existing crystal, fossil, or object.
- CathodoluminescenceLuminescence
- Light emitted when a material is bombarded by electrons, widely used to reveal growth zones and textures.
- CationMineral chemistry
- A positively charged ion. Common mineral cations include sodium, calcium, magnesium, iron, and aluminum.
- Cauliflower habitHabit & formCollector
- A collector-friendly description for rounded, knobby clusters resembling a head of cauliflower. It is descriptive rather than a formal form.
- CavityGeologic occurrence
- An open space within a rock or mineral mass where crystals may grow with fewer constraints.
- ChalcedonyCommon minerals
- A microcrystalline to cryptocrystalline form of silica made of very fine quartz and related silica structures.
- ChalcopyriteCommon minerals
- A brass-yellow copper iron sulfide, CuFeS2, and a major copper ore mineral.
- Channel sampleField methods
- A continuous or regularly collected sample taken along a cut channel across rock or mineralization.
- ChatoyancyOptical phenomena
- A moving band of reflected light resembling a cat’s eye, produced by aligned fibers, channels, or inclusions.
- Chemical analysisAnalytical methods
- Measurement of the elements, ions, or compounds present in a mineral and their amounts.
- Chemical bondMineral chemistry
- The force holding atoms or ions together. Ionic, covalent, metallic, and weaker bonds influence mineral properties.
- Chemical cleaningSpecimen care
- Cleaning a specimen with acids, bases, reducing agents, oxidizers, chelating agents, or other chemicals.
- Chip sampleField methods
- Small rock chips collected systematically across an exposure, vein, or interval.
- ChippedCollectingCollector
- Having a small piece broken from an edge, corner, or termination.
- ChloritizationAlteration
- Alteration in which chlorite forms at the expense of earlier minerals.
- ChromophoreColor & optics
- An element, ion, or structural feature responsible for producing color in a mineral or gem.
- CitrineCommon mineralsVarietal
- A yellow to orange or brownish-yellow variety of quartz. Much commercial citrine is heat-treated amethyst.
- ClaimMining
- A legal right or interest to explore for and potentially extract minerals from a defined area under applicable law.
- Clast-supportedPetrology
- Describing a sedimentary rock in which larger clasts touch one another and form the main supporting framework.
- CleavagePhysical properties
- The tendency of a mineral to split along specific planes of weakness controlled by its crystal structure.
- Cloud inclusionInclusionsGemological
- A hazy region made of numerous microscopic inclusions too small to resolve individually.
- CloudyCollectingCollector
- Containing fine inclusions, microfractures, bubbles, or scattering features that reduce transparency.
- ClusterCollectingCollector
- An informal collector term for a natural group of crystals attached to one another or to shared matrix.
Also: Crystal cluster
- Coated gemTreatmentsTrade
- A gem material with a thin artificial surface layer added to modify color, luster, or optical appearance.
- CoatingHabit & form
- A thin mineral layer deposited on another material. A coating may be uniform, patchy, crystalline, or powdery.
- Cockscomb habitHabit & form
- A crest-like aggregate of bladed crystals resembling a rooster’s comb. Barite and calcite may show this habit.
- ColloformHabit & form
- A broad descriptive term for rounded, layered, or banded growth surfaces produced by successive deposition.
- ColorPhysical properties
- The visible hue of a mineral in reflected or transmitted light. Color can be diagnostic for some minerals but highly variable in others.
- Color changeOptical phenomena
- A noticeable difference in perceived body color under different kinds of illumination. Alexandrite is the classic mineral example.
- Color zoningGrowth features
- Variation in color between successive regions of a crystal. Tourmaline and fluorite often show striking examples.
- ColumnarHabit & form
- Formed as stout, elongated columns or column-like crystals.
- CompactHabit & form
- Dense and tightly packed, with little visible pore space or obvious crystal separation.
- Complete crystalCollectingCollector
- A collector term for a crystal with most or all expected natural faces preserved and little attachment damage.
- Composite specimenCollectingCollector
- A specimen assembled from two or more pieces. Responsible sellers disclose composites.
- Composition planeTwinning
- The surface along which two individuals of a contact twin are joined.
- Conchoidal fracturePhysical properties
- A smooth, curved fracture resembling the inside of a shell. Quartz and volcanic glass commonly show it.
- ConcretionGeologic occurrence
- A compact mass formed by mineral cement precipitating around a nucleus within sediment or sedimentary rock.
- ContactCollectingCollector
- An area where a crystal grew against another crystal or matrix, preventing a free crystal face from developing.
- Contact markCollectingCollector
- A flattened, rough, or incomplete area showing where a crystal was attached during growth.
- Contact metamorphismGeologic occurrence
- Metamorphism caused mainly by heating and fluid interaction near an igneous intrusion.
- Contact twinTwinning
- A twin in which two crystal individuals meet along a distinct composition surface.
- CoralloidHabit & form
- Branching or knobby in a way that resembles coral. Calcite and aragonite can form coralloid aggregates.
- Core sampleField methods
- A section of cylindrical material recovered by drilling and retained for logging, testing, or analysis.
Also: Drill core
- CorundumCommon minerals
- Aluminum oxide, Al2O3, a very hard mineral with gem varieties ruby and sapphire.
- CotypeClassificationHistorical
- An older term for one of several specimens used in the original study of a mineral when no single holotype was designated.
- Country rockGeologic occurrence
- The pre-existing rock surrounding an intrusion, vein, or mineral deposit. It may be altered by heat or mineralizing fluids.
- Coupled substitutionMineral chemistry
- A pair or group of substitutions that occur together so electrical charge remains balanced.
- Covalent bondMineral chemistry
- A strong bond formed when atoms share electrons.
- CrosscutMining
- A horizontal underground passage driven across rock structure or mineralization.
- Crossed polarsOptical mineralogy
- A microscope condition in which the polarizer and analyzer are oriented at 90 degrees.
Also: Crossed nicols
- CrownLapidary
- The portion of a faceted gemstone above the girdle.
- CrustHabit & form
- A mineral layer covering a surface, often formed by deposition, weathering, evaporation, or alteration.
- CrystalCrystallography
- A solid whose atoms, ions, or molecules are arranged in an ordered repeating pattern. A well-formed crystal may show external faces that reflect this internal order.
- Crystal axisCrystallography
- An imaginary reference direction used to describe crystal geometry and symmetry.
- Crystal castCollectingCollector
- A replica of a former crystal produced when another mineral fills or coats the space it occupied.
Also: Cast
- Crystal caveCollector terminologyCollector
- An informal term for a cave or cavity notable for abundant crystal growth.
- Crystal cavernCollector terminologyCollector
- An informal term for a large cavity or cave whose walls are lined with abundant crystals.
- Crystal classCrystallography
- One of the 32 possible combinations of crystallographic point symmetry.
- Crystal clusterCollectingCollector
- A group of two or more crystals naturally attached and grown together.
- Crystal crustCollectingCollector
- A surface covered by a continuous or nearly continuous layer of crystals.
Also: Druzy
- Crystal edgeCrystallography
- The line where two crystal faces meet.
- Crystal faceCrystallography
- A flat external surface on a well-developed crystal. Its orientation reflects the internal structure.
- Crystal formCrystallography
- A set of crystal faces related by symmetry. In casual collecting, “form” is often used more loosely for overall shape.
- Crystal habitHabit & form
- The characteristic overall shape or growth style in which a mineral commonly occurs. Habits include prismatic, tabular, acicular, fibrous, botryoidal, and many others.
- Crystal latticeCrystallography
- A periodic geometric array representing repeating positions in a crystal.
- Crystal liningCollectingCollector
- A layer of crystals covering the interior wall of a cavity, geode, or vug.
- Crystal pocketCollectingCollector
- An open cavity containing well-developed crystals, commonly encountered in pegmatites, veins, and volcanic rocks.
Also: Pocket
- Crystal sprayCollectingCollector
- A cluster of slender crystals radiating or fanning from a common base.
Also: Spray
- Crystal structureCrystallography
- The three-dimensional arrangement of atoms, ions, or molecules within a crystal. Structure strongly influences cleavage, hardness, symmetry, and optical behavior.
- Crystal systemCrystallography
- One of the seven major symmetry-based crystal categories: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic.
- Crystal vertexCrystallography
- A point where three or more crystal faces or edges meet.
- CrystallineCrystallography
- Having an ordered internal atomic structure. A material can be crystalline even when it does not show visible crystal faces.
- CubeCrystallography
- A six-faced crystal form with square faces meeting at right angles. Halite and fluorite commonly form cubes.
- Cubic cleavagePhysical properties
- Cleavage in three directions at roughly 90 degrees, producing cube-like fragments. Halite is a classic example.
- Cubic systemCrystallography
- The crystal system with three equal axes at right angles. Halite, fluorite, pyrite, and garnet are examples.
Also: Isometric system
- CuletLapidary
- The point or small facet at the bottom of a gemstone’s pavilion.
- Cyclic twinningTwinning
- Repeated twinning arranged around an axis so multiple individuals form a ring, star, or cyclic aggregate.
- CyclosilicateMineral classes
- A silicate in which tetrahedra are linked into rings. Beryl and tourmaline are examples.
Also: Ring silicate
D
- DamageCollectingCollector
- Breakage, bruising, chipping, abrasion, or other loss affecting a specimen. Collectors often distinguish natural contacts from later damage.
- Daughter crystalInclusions
- A tiny crystal that precipitated inside a fluid inclusion after the inclusion was trapped.
- DeclineMining
- An inclined underground mine opening driven downward from the surface or another level.
- DehydrationMineral chemistry
- Loss of water or hydroxyl from a mineral during heating, weathering, or reaction.
- Dehydration crackingSpecimen care
- Cracking or disintegration caused when a hydrated mineral loses water or experiences damaging humidity changes.
- DeliquescentSpecimen care
- Describing a substance that absorbs enough moisture from the air to dissolve in the absorbed water.
- DendriticHabit & form
- Branching in tree-like or fern-like patterns. Dendrites may be true mineral growths or surface deposits along fractures.
- DensityPhysical properties
- Mass per unit volume, commonly expressed for minerals in grams per cubic centimeter.
- Desert roseHabit & formCollector
- A rosette-shaped aggregate, usually gypsum or barite, containing sand grains incorporated during growth.
- DiagenesisGeologic occurrence
- Physical and chemical changes affecting sediment after deposition and during conversion into sedimentary rock, before metamorphism.
- DiaphaneityPhysical properties
- The general degree of light transmission through a mineral, commonly expressed as transparent, translucent, or opaque.
Also: Transparency
- DichroismOptical properties
- Pleochroism showing two principal colors, typical of many uniaxial colored crystals.
- DimorphMineral chemistry
- Either member of a pair of polymorphs with the same composition but two different crystal structures.
- DipyramidCrystallography
- A closed crystal form resembling two pyramids joined base to base.
- Discredited mineralClassification
- A former mineral name no longer accepted as a valid mineral species, often because later work showed it was a mixture, synonym, or otherwise invalid.
- DispersionOptical properties
- The separation of white light into colors because refractive index varies with wavelength.
- Display specimenCollectingCollector
- A mineral specimen chosen mainly for aesthetic or exhibition quality.
- Disseminated mineralizationEconomic geology
- Ore or other minerals distributed as many small grains through a larger volume of rock rather than concentrated in one vein.
- Dissolution featureGrowth features
- A pit, channel, rounding, or other surface form produced when part of a crystal dissolves after growth.
- DivergentHabit & form
- Crystals spreading apart from a common base or center rather than remaining parallel.
- DodecahedronCrystallography
- A twelve-faced crystal form; the rhombic dodecahedron is especially common in garnet.
- Dogtooth sparHabit & formHistorical
- A traditional term for sharply pointed scalenohedral calcite crystals resembling canine teeth.
Also: Dogtooth calcite
- DolomiteCommon minerals
- A carbonate mineral with ideal formula CaMg(CO3)2. Mineral dolomite commonly forms rhombohedral crystals.
- DolostonePetrology
- A carbonate sedimentary rock composed mainly of the mineral dolomite.
- Double refractionOptical properties
- Visible splitting of an image into two because light separates into two rays in an anisotropic crystal. Clear calcite can show this dramatically.
Also: Birefringent doubling
- Double-terminatedCollectingCollector
- Having natural crystal terminations at both ends, usually because the crystal grew with open space around both ends.
- DriftMining
- A roughly horizontal underground passage driven along a vein, orebody, or mine level.
- Drill coreField methods
- A cylindrical sample of rock recovered by drilling that preserves a subsurface record.
- DruseHabit & form
- A cavity or surface lined with numerous small crystals. The term may refer to the cavity or crystalline coating itself.
Also: Drusy, Druze
- DruzyHabit & form
- A sparkling coating of many tiny crystals covering a surface or cavity. “Drusy” is another common spelling.
Also: Drusy, Druze
- DuctilePhysical properties
- Capable of being drawn into wire. Native gold, silver, and copper are ductile.
- Dull lusterLuster
- A surface with little obvious reflection, often because the material is very fine-grained, weathered, or porous.
Also: Earthy luster
- DyedTreatmentsTrade
- Artificially colored by introducing dye into pores, fractures, or surface layers.
E
- EarthyHabit & form
- Fine-grained, dull, and soil-like in appearance or texture.
- Earthy fracturePhysical properties
- A dull, crumbly break resembling compact soil or clay.
- Earthy lusterLuster
- A very low, matte luster resembling dry clay or soil.
- EDSAnalytical methods
- Energy-dispersive X-ray spectroscopy, commonly attached to an SEM for rapid elemental analysis of small areas.
Also: EDX, Energy-dispersive spectroscopy
- EffervescenceIdentification
- Visible bubbling caused by gas release during a chemical reaction. Calcite typically effervesces readily in dilute hydrochloric acid.
- EfflorescenceSpecimen care
- A powdery or crystalline surface deposit formed when soluble salts migrate and crystallize as water evaporates.
- ElasticPhysical properties
- Able to bend and then return toward the original shape after the force is removed. Thin mica sheets are a familiar example.
- Electron microprobeAnalytical methods
- An instrument that measures the chemical composition of tiny spots on polished samples using an electron beam and emitted X-rays.
Also: EPMA
- ElementMineral chemistry
- A substance whose atoms all have the same number of protons.
- EncrustingHabit & form
- Forming a crust or coating over another mineral, rock, or cavity surface.
- End-memberMineral chemistry
- An idealized composition at the extreme end of a solid-solution series. Natural minerals often lie between two or more end-members.
- EnhydroCollector terminologyCollector
- A collector term for a specimen containing a fluid-filled cavity, sometimes with a movable gas bubble.
- Enhydro agateCollector terminologyCollector
- Agate containing a fluid-filled cavity, sometimes with a movable bubble.
- Enrichment zoneEconomic geology
- A zone where metals become concentrated by secondary processes, commonly beneath an oxidation zone.
- EpimorphAlteration
- A hollow shell or crust that preserves the external shape of a crystal that was later removed or dissolved away.
- EpitaxyCrystallography
- Oriented growth of one crystalline material on another because their surface structures have a favorable geometric match.
- EpithermalGeologic occurrence
- Describing relatively shallow hydrothermal mineralization formed from hot fluids at low to moderate temperatures.
- EPMAAnalytical methods
- Electron probe microanalysis, commonly performed with an electron microprobe for quantitative elemental analysis.
Also: Electron probe microanalysis
- Essential mineralPetrology
- A mineral whose presence is important to the classification or identity of a particular rock.
- Etched crystalGrowth features
- A crystal whose original surfaces have been partly dissolved, producing pits, channels, matte areas, or complex patterns.
- EuhedralHabit & form
- Having well-developed crystal faces because the crystal had room to grow freely.
- EvaporiteGeologic occurrence
- A mineral or sedimentary deposit formed by evaporation of water and precipitation of dissolved salts.
- ExposureField methods
- Any place where rock or a geologic unit is visible at the surface.
- ExsolutionMineral chemistry
- The unmixing of an originally homogeneous solid solution into two or more distinct phases as conditions change.
- Exsolution lamellaeMineral chemistry
- Thin, often oriented layers of a second mineral phase formed by exsolution.
- ExtinctionOptical properties
- The dark position reached by an anisotropic mineral between crossed polarizers when its vibration directions align with the polarizers.
F
- FabricPetrology
- The spatial arrangement and preferred orientation of mineral grains, crystals, or structural features in a rock.
- FacetLapidary
- A flat polished surface intentionally cut on a gemstone.
- FacetingLapidary
- Cutting and polishing a gemstone into a planned arrangement of flat facets.
- Fake specimenCollectingCollector
- An informal term for a specimen that has been fabricated, misrepresented, or artificially altered without disclosure.
- Fan-shapedHabit & form
- An aggregate in which elongated crystals diverge outward in a flattened fan.
- Feather inclusionInclusionsGemological
- A gemological term for a fracture or group of fractures that appears feather-like under magnification.
- FeelField methods
- A tactile property sometimes useful in field identification, such as the greasy feel of talc.
- FeldsparCommon minerals
- A major group of framework silicate minerals that dominate much of Earth’s crust.
- FibrousHabit & form
- Composed of slender, thread-like crystals or fibers. Some fibrous minerals can be hazardous when airborne.
- Field cleaningSpecimen care
- Basic removal of loose dirt or unstable matrix at the collecting site.
- Field collectedCollectingCollector
- Collected directly at a natural exposure, mine, quarry, dump, or other locality rather than purchased from another collector or dealer.
- Field specimenCollecting
- A specimen collected primarily to document the geology or mineralogy of a locality.
- FiliformHabit & form
- Wire-like or thread-like, especially for thin curved metallic crystals such as some native silver.
- Fingerprint inclusionInclusionsCollector
- A curved or irregular pattern of tiny fluid inclusions resembling a fingerprint.
- FireGemology
- In gemology, flashes of spectral color produced mainly by dispersion in a cut stone.
Also: Dispersion fire
- FlexiblePhysical properties
- Able to bend without breaking but not necessarily return fully to the original shape.
- FloatField methods
- Loose rock or mineral material that has moved from its original bedrock source.
- Floater crystalCollectingCollector
- A crystal or cluster that grew free or nearly free of matrix and is naturally crystallized on most or all sides.
Also: Floater
- Fluid inclusionInclusions
- A microscopic pocket of liquid, gas, or both trapped within a crystal.
- FluorescenceLuminescence
- Visible light emitted while a material is excited by higher-energy radiation such as ultraviolet light. The glow usually stops almost immediately when excitation stops.
- FluoriteCommon minerals
- Calcium fluoride, CaF2, a cubic mineral famous for colorful cubes and octahedra, strong cleavage, and fluorescence in many specimens.
- FoliatedHabit & form
- Made of thin layers, leaves, or sheets. The term is used for mineral aggregates as well as metamorphic rock textures.
- FoliationPetrology
- A planar fabric in a rock produced by alignment, segregation, or flattening of minerals during metamorphism or deformation.
- FormulaMineral chemistry
- A compact chemical expression showing the elements and their proportions in a mineral.
- FracturePhysical properties
- The way a mineral breaks when it does not split along cleavage or parting planes.
- Frosted crystalCollectingCollector
- A collector description for a crystal surface with a matte or finely textured appearance.
- FTIRAnalytical methods
- Fourier-transform infrared spectroscopy, commonly used for mineral and gem identification and treatment detection.
Also: Fourier-transform infrared spectroscopy
- FumaroleGeologic occurrence
- A volcanic vent emitting hot gases and vapors.
G
- GalenaCommon minerals
- Lead sulfide, PbS, a very dense metallic mineral with perfect cubic cleavage.
- GangueEconomic geology
- Minerals or rock that accompany valuable ore minerals but are not the principal material being mined. Quartz and calcite commonly occur as gangue in veins.
- GarnetCommon minerals
- A group of cubic silicate minerals with related structures and varied compositions.
- Gas inclusionInclusions
- A gas-filled or gas-rich cavity enclosed within a crystal.
- GemGemology
- A mineral, mineraloid, rock, or organic material valued for beauty, rarity, or durability and suitable for ornament after cutting or polishing.
- GemmyCollectingCollector
- An informal collector term for a crystal or part of a crystal with unusually good transparency and gem-like appearance.
- GemstoneGemology
- A natural or manufactured gem material used for jewelry or ornament. In mineral collecting, the term usually refers to natural gem-quality material.
- GeodeGeologic occurrence
- A hollow or partly hollow rock cavity lined with crystals or mineral layers.
- GirdleLapidary
- The narrow perimeter separating the crown from the pavilion on a faceted gemstone.
- GlobularHabit & form
- Made of rounded, ball-like mineral forms that may occur singly or in clusters.
- GneissPetrology
- A coarse metamorphic rock with compositional banding or gneissic foliation.
- GoethiteCommon minerals
- Iron oxyhydroxide, FeO(OH), common in weathering and oxidation zones and often botryoidal or fibrous.
- Good cleavagePhysical properties
- Clearly developed cleavage that is easy to recognize but less effortless or smooth than perfect cleavage.
- GossanEconomic geology
- A rusty, iron-rich weathered cap developed over some sulfide deposits after oxidation and leaching.
- Grab sampleField methods
- A sample collected from a convenient spot rather than through a strictly systematic sampling pattern.
- GrainFundamentals
- An individual crystal or crystal fragment within a rock, sediment, or mineral aggregate. Grains may be microscopic or easily visible.
- Grandfathered mineralClassification
- A mineral name accepted because it was in established use before modern mineral-approval procedures.
- GranitePetrology
- A coarse-grained intrusive igneous rock rich in quartz and feldspar, commonly with mica or amphibole.
- GranularHabit & form
- Made of many roughly equidimensional mineral grains.
- Greasy lusterLuster
- A surface appearance that seems coated with a thin film of oil or grease.
- GreisenGeologic occurrence
- A hydrothermally altered granitic rock rich in quartz and mica, commonly associated with tin and tungsten mineralization.
- GroundmassPetrology
- The finer-grained material surrounding larger crystals or fragments in a rock.
- Growth hillockGrowth features
- A small raised feature on a crystal face produced as layers advance during crystal growth.
- Growth striationGrowth features
- Fine parallel lines or grooves on a crystal face created by repeated growth steps or alternating faces.
- Growth zoningGrowth features
- Visible or measurable changes from one growth layer to another within a crystal.
- GypsumCommon minerals
- Hydrated calcium sulfate, CaSO4·2H2O, a very soft mineral that can form clear crystals, fibrous satin spar, alabaster, and desert roses.
H
- Hackly fracturePhysical properties
- A jagged fracture with sharp, torn-looking projections. Native copper can show hackly fracture.
- HairlikeHabit & formCollector
- Extremely fine, threadlike crystals. The term is descriptive rather than a strict crystallographic category.
- HalideMineral classes
- A mineral dominated by halogen anions such as chloride or fluoride. Halite and fluorite are examples.
- Hand lensField methods
- A small magnifying lens, commonly 10×, used in the field to examine crystals, grains, cleavage, inclusions, and textures.
- Hand specimenCollecting
- A piece of rock or mineral small enough to hold and examine directly.
- HardnessPhysical properties
- A mineral’s resistance to scratching. Mineralogical hardness is commonly compared using the Mohs scale.
- Heat sensitiveSpecimen care
- Describing a mineral or specimen that may crack, dehydrate, alter, fade, or otherwise change when heated.
- Heat treatedTreatmentsTrade
- Intentionally heated to change color, clarity, inclusions, or other properties.
- HeftCollectingCollector
- An informal collector’s impression of how heavy a specimen feels for its size.
- HematiteCommon minerals
- Iron oxide, Fe2O3, occurring in metallic, earthy, platy, botryoidal, and other forms. Its streak is typically reddish brown.
- Herkimer diamondCollector terminologyTrade
- A trade and locality term for unusually clear, commonly double-terminated quartz crystals from the Herkimer region of New York. They are quartz, not diamond.
- Hexagonal systemCrystallography
- A crystal system based on a sixfold symmetry axis, commonly represented with three equivalent horizontal axes and one vertical axis.
- HolotypeClassification
- The single specimen designated as the primary type for a new mineral species when the species is described.
- Hopper crystalHabit & form
- A crystal whose edges and corners grew faster than the centers of its faces, producing recessed or stepped faces.
- Host rockGeologic occurrence
- The rock that contains a mineral deposit, vein, inclusion, or other feature of interest.
- Hourglass zoningGrowth features
- A sector-zoning pattern in which differently colored or composed regions form an hourglass-like cross section.
- Humidity sensitiveSpecimen care
- Describing a mineral whose stability or appearance can be affected by moisture in the air.
- HydrationMineral chemistry
- Incorporation of water molecules or hydroxyl into a material through reaction or structural change.
- HydrothermalGeologic occurrence
- Related to hot water-rich fluids circulating through rocks. These fluids can dissolve, transport, and redeposit minerals.
- Hydrothermal alterationAlteration
- Chemical and mineralogical change caused by interaction with hot fluids.
- Hydrothermal brecciaGeologic occurrence
- Broken rock produced or modified by pressurized hydrothermal fluids and commonly cemented by later minerals.
- Hydrothermal veinGeologic occurrence
- A fracture or fissure filled by minerals precipitated from hot aqueous fluids.
- HydroxideMineral classes
- A mineral containing hydroxyl groups as an essential anionic component.
- HygroscopicSpecimen care
- Describing a material that readily absorbs moisture from the air.
- HypogeneEconomic geology
- Formed by ascending or deep-seated geologic fluids and processes below the near-surface weathering zone.
- HypothermalGeologic occurrenceHistorical
- An older term for high-temperature, relatively deep hydrothermal mineralization.
I
- Ideal formulaMineral chemistry
- A simplified chemical formula representing the end-member or theoretical composition of a mineral. Natural specimens commonly contain substitutions and impurities.
- IdiochromaticColor & optics
- Describing a mineral whose characteristic color is caused by elements essential to its chemical formula.
- IgneousGeologic occurrence
- Formed from the cooling and solidification of magma or lava.
- Igneous rockPetrology
- Rock formed by cooling and solidification of molten material. Granite and basalt are common examples.
- ImitationTreatments
- A material made to resemble another gem or mineral without having the same composition and structure.
Also: Simulant
- ImpurityMineral chemistry
- A minor component not required by the ideal chemical formula.
- In situField methods
- In its original place or geologic position.
- Included crystalGrowth features
- A crystal containing solid, liquid, or gas inclusions trapped during or after growth.
- Included quartzCollector terminologyCollector
- Quartz containing visible inclusions of minerals, fluids, gases, or other materials.
- InclusionInclusions
- Material trapped within a mineral or gem. Inclusions may be solid crystals, glass, liquid, gas, or combinations of these.
- Infrared spectroscopyAnalytical methods
- A technique that studies how a material absorbs infrared light, revealing molecular vibrations and functional groups.
Also: IR spectroscopy, FTIR
- InosilicateMineral classes
- A silicate built from chains of linked tetrahedra. Pyroxenes have single chains and amphiboles double chains.
Also: Chain silicate
- IntaglioLapidary
- An engraved design cut below the surface of a gem or other material.
- Interference colorOptical properties
- Color produced when polarized light rays traveling at different speeds through a birefringent material recombine and interfere.
- Interference figureOptical mineralogy
- An optical pattern observed in convergent polarized light that helps determine optic character and orientation.
- IntergrowthGrowth features
- Two or more crystals or minerals grown together in an intimate geometric or irregular relationship.
- IonMineral chemistry
- An atom or group of atoms carrying a net electrical charge because electrons have been gained or lost.
- Ionic bondMineral chemistry
- Electrostatic attraction between oppositely charged ions.
- IridescenceOptical phenomena
- A shifting rainbow-like color effect caused by interference, diffraction, or thin films.
- Iris quartzCollector terminologyTrade
- A collector or gem term for quartz showing rainbow interference colors from internal fractures or related effects.
- IrradiatedTreatmentsTrade
- Exposed intentionally to radiation to alter color or other properties.
- Isometric systemCrystallography
- Another name for the cubic crystal system.
Also: Cubic system
- Isomorphous substitutionMineral chemistry
- Substitution of chemically similar ions into equivalent structural sites without changing the basic structure.
- IsotropicOptical properties
- Having the same optical behavior in all directions. Cubic minerals and amorphous materials are optically isotropic.
J
- JasperCommon minerals
- An opaque, commonly iron-colored microcrystalline silica-rich material used as a gem and ornamental stone.
K
- KaolinizationAlteration
- Alteration of feldspars and other aluminosilicates to kaolinite-rich clay.
- KaratGemology
- A measure of gold purity expressed in 24 parts. It is different from the gemstone mass unit carat.
- KarstGeologic occurrence
- Terrain produced mainly by dissolution of soluble rocks such as limestone and dolostone.
- KyaniteCommon minerals
- An aluminum silicate commonly forming bladed crystals and showing strongly direction-dependent hardness.
L
- LabelCollecting
- A written or printed record accompanying a specimen, ideally documenting identification, locality, collector, date, and collection history.
- LabradorescenceOptical phenomena
- A broad shifting display of blue, green, gold, or other colors produced by interference within fine structures in labradorite and related feldspars.
- LamellarHabit & form
- Composed of thin plates or layers arranged in a sheet-like aggregate.
- LapidaryLapidary
- A person who cuts, shapes, engraves, or polishes gemstones and ornamental stone, or the craft of doing so.
- LateriteGeologic occurrence
- Highly weathered, iron- and aluminum-rich material formed in warm, humid climates by intense leaching.
- LatticeCrystallography
- A repeating array of points used to describe the periodicity of a crystal.
- LavaGeologic occurrence
- Magma that has reached Earth’s surface.
- LevelMining
- A set of underground mine workings at approximately the same elevation.
- Light sensitiveSpecimen care
- Describing a mineral or gem whose color or condition can change significantly with exposure to light.
- LimestonePetrology
- A sedimentary rock composed mainly of calcium carbonate, commonly as calcite or aragonite-derived material.
- LimoniteCommon mineralsHistorical
- A field and historical term for mixtures of hydrated iron oxides and oxyhydroxides, commonly dominated by goethite. It is not a single mineral species.
- LineationPetrology
- A linear fabric defined by aligned minerals, stretched objects, fold axes, or other directional features.
- LocalityCollecting
- The geographic place from which a mineral specimen was collected. Good locality data can be scientifically and historically important.
- Locality specimenCollectingCollector
- A specimen valued partly because it represents a particular mine, district, region, or classic occurrence.
- LodeEconomic geology
- A mineralized vein, zone, or body traditionally considered a source of ore.
- Long-prismaticHabit & form
- Strongly elongated prismatic crystals whose length greatly exceeds their width.
- Longwave UVLuminescence
- Ultraviolet radiation in the longer-wavelength UV-A range, commonly around 365 nm in mineral lamps.
- LoupeField methods
- A compact magnifier used by mineral collectors, gemologists, and jewelers.
- LuminescenceLuminescence
- Emission of light not caused simply by high temperature. Fluorescence and phosphorescence are examples.
- LusterLuster
- The way light reflects from a mineral surface. Common lusters include metallic, vitreous, adamantine, pearly, silky, resinous, greasy, waxy, and dull.
- LustrousCollectingCollector
- Having strong, attractive surface reflection. In collector descriptions, the term is usually relative.
M
- MacrocrystalCollecting
- A crystal large enough to be seen clearly without a microscope.
- MagmaGeologic occurrence
- Molten or partly molten rock beneath Earth’s surface, commonly containing crystals and dissolved gases.
- MagnetismPhysical properties
- A mineral’s response to a magnetic field. Magnetite is strongly magnetic, while many minerals respond only weakly.
- MagnetiteCommon minerals
- Iron oxide, Fe3O4, commonly black and strongly magnetic.
- MalachiteCommon minerals
- A green copper carbonate hydroxide mineral, Cu2CO3(OH)2, commonly botryoidal, fibrous, banded, or stalactitic.
- MalleablePhysical properties
- Capable of being hammered or pressed into thin sheets without breaking. Native gold and copper are strongly malleable.
- MammillaryHabit & form
- Formed as large, smooth, rounded bulges resembling broad domes.
- MarblePetrology
- A metamorphic rock formed mainly by recrystallization of limestone or dolostone.
- MarcasiteCommon minerals
- Iron disulfide, FeS2, with an orthorhombic structure distinct from cubic pyrite. It can be unstable in humid collections.
- Marcasite decaySpecimen care
- Oxidative deterioration of marcasite, often producing acidic products and structural breakdown.
- MassiveHabit & form
- Occurring as a compact mass without obvious individual crystal faces. “Massive” describes form, not specimen size.
- MatrixCollecting
- The rock or mineral material on which crystals or another mineral of interest occur. Collectors often value specimens that preserve attractive natural matrix.
- Matrix-supportedPetrology
- Describing a rock or sediment in which larger grains or clasts are supported mainly by finer matrix.
- Mechanical cleaningSpecimen care
- Removal of matrix or dirt with tools, brushes, picks, saws, air scribes, or abrasives.
- MesothermalGeologic occurrenceHistorical
- An older broad term for hydrothermal deposits interpreted to have formed at intermediate temperatures and depths.
- Metallic bondMineral chemistry
- Bonding in which electrons are shared broadly among many atoms, giving metals conductivity and ductility.
- Metallic lusterLuster
- A bright, metal-like reflective appearance typical of opaque minerals such as galena and pyrite.
- MetamorphicGeologic occurrence
- Formed or changed by metamorphism.
- Metamorphic rockPetrology
- Rock transformed in the solid state by heat, pressure, deformation, and fluids.
- MetamorphismGeologic occurrence
- Mineralogical and textural change in solid rock caused by heat, pressure, stress, and reactive fluids without complete melting.
- MetasomatismGeologic occurrence
- Chemical alteration of a rock by fluids that add or remove components while the rock remains largely solid.
- MicaCommon minerals
- A group of sheet silicate minerals with excellent basal cleavage that allows splitting into thin sheets.
- MicaceousHabit & form
- Splitting or occurring in very thin, sheet-like flakes resembling mica.
- MicrocrystalCollecting
- A crystal that is microscopic or best viewed under magnification.
- MicromountCollectingCollector
- A very small mineral specimen mounted for viewing under magnification.
- Milky quartzCommon mineralsVarietal
- A white to cloudy variety of quartz, commonly caused by abundant microscopic fluid inclusions.
- Miller indicesCrystallography
- A set of integers used to specify the orientation of crystal planes or faces, written in forms such as (hkl).
- MineMining
- An excavation or operation developed to extract minerals or other geologic materials.
- Mine dumpCollectingCollector
- Rock discarded from mine workings and piled at the surface. Historic dumps can be productive collecting sites where access is legal.
Also: Dump
- MineralFundamentals
- A naturally occurring solid with an ordered internal structure and a characteristic chemical composition or compositional range. Most minerals are inorganic, although modern mineral classification recognizes limited exceptions under specific criteria.
- Mineral groupClassification
- A set of related mineral species sharing important structural or chemical features. Feldspars, garnets, and micas are familiar mineral groups.
- Mineral identificationIdentification
- The process of determining a mineral’s identity using physical properties, crystallography, chemistry, optical behavior, or analytical methods.
- Mineral seriesClassification
- A sequence of mineral compositions linked by continuous or near-continuous chemical substitution. The plagioclase series runs from albite-rich to anorthite-rich compositions.
- Mineral speciesClassification
- A distinct mineral recognized by its essential chemical composition and crystal structure. Quartz, calcite, and pyrite are mineral species.
- Mineral specimenCollecting
- A collected example of one or more minerals preserved for scientific study, education, or display.
- Mineral varietyClassification
- A named form of a mineral species distinguished by color, composition, habit, or another non-essential feature. Amethyst is a purple variety of quartz.
- MineraloidFundamentals
- A naturally occurring mineral-like material that lacks the long-range ordered crystal structure required of a mineral. Opal and obsidian are commonly described as mineraloids.
- Miniature specimenCollectingCollector
- A collector-size category larger than a thumbnail and smaller than a cabinet specimen.
- Mohs scalePhysical properties
- A relative scale of scratch hardness from 1 to 10, using talc as 1 and diamond as 10. The numerical steps are not equal intervals of absolute hardness.
Also: Mohs hardness scale
- Mohs testIdentification
- A scratch test comparing an unknown mineral with materials of known hardness.
- MoldGeologic occurrence
- A cavity or impression preserving the external shape of a former crystal, fossil, or object.
- MolybdateMineral classes
- A mineral containing molybdate groups, commonly MoO4. Wulfenite is an example.
- Monoclinic systemCrystallography
- A crystal system with three unequal axes in which one axis is inclined relative to the other two in the conventional setting.
- Moving bubble inclusionInclusionsCollector
- A fluid inclusion with a gas bubble that visibly moves as the specimen is rotated.
- Museum specimenCollectingCollector
- A specimen of exceptional scientific, historical, aesthetic, or locality importance suitable for institutional display or study.
N
- Nailhead sparHabit & formHistorical
- A traditional term for calcite crystals dominated by broad rhombohedral faces, giving a blunt nail-head appearance.
- Native elementMineral classes
- A mineral composed principally of a single chemical element. Native gold, copper, sulfur, and carbon minerals are examples.
- Needle inclusionInclusions
- A slender, needle-like crystal trapped inside a host mineral.
- Negative crystalInclusions
- A cavity inside a crystal whose shape is controlled by the host crystal’s structure and resembles an inward-facing crystal form.
- NesosilicateMineral classes
- A silicate whose structure is based on isolated SiO4 tetrahedra. Olivine and garnet are examples.
Also: Orthosilicate
- NitrateMineral classes
- A mineral containing nitrate groups, NO3, as essential components.
- NoduleGeologic occurrence
- A rounded or irregular mineral or rock mass that developed within a host material, commonly sedimentary rock.
O
- Obsolete nameNomenclatureHistorical
- A mineral name no longer recommended in modern scientific usage but sometimes preserved in historical literature or old collections.
- Octahedral cleavagePhysical properties
- Cleavage parallel to octahedral planes, commonly producing triangular faces or octahedron-like fragments. Fluorite and diamond are examples.
- OctahedronCrystallography
- An eight-faced form commonly consisting of triangular faces. Fluorite, diamond, and magnetite may form octahedra.
- OdorField methods
- A smell produced naturally or when a mineral is rubbed or broken. Odor is only occasionally useful in identification.
- OiledTreatmentsTrade
- Treated with oil to improve appearance, transparency, or surface luster.
- OpalCommon minerals
- Hydrated amorphous to poorly crystalline silica. Precious opal displays play-of-color; common opal does not.
- OpalescenceOptical phenomena
- A milky or glowing appearance caused by light scattering. It should not automatically be confused with opal play-of-color.
- OpaquePhysical properties
- Not transmitting visible light through an ordinary thickness of the material.
- Open-pit mineMining
- A surface mine excavated as a large open pit, typically with stepped benches.
- Optic axisOptical properties
- A direction in an anisotropic crystal along which light does not experience the usual birefringent separation.
- OreEconomic geology
- Rock or mineralized material containing a commodity in sufficient concentration that extraction may be economically worthwhile under the conditions considered.
- Ore mineralEconomic geology
- A mineral from which a useful element or commodity is extracted. Chalcopyrite is an important copper ore mineral.
- Organic mineralMineral classes
- A recognized mineral species whose essential composition includes organic chemical groups and that meets mineralogical criteria.
- OrientOptical phenomena
- A soft iridescent sheen, especially associated with pearls but sometimes used for other gem materials.
- Orthorhombic systemCrystallography
- A crystal system with three mutually perpendicular axes of different lengths.
- Oscillatory zoningGrowth features
- Repeated bands of changing composition produced as growth conditions fluctuate.
- OutcropField methods
- An exposure of bedrock at Earth’s surface.
- OvergrowthGrowth features
- A later generation of the same or a different mineral that grows on the surface of an earlier crystal.
- OxidationAlteration
- A chemical process involving loss of electrons, commonly associated near the surface with reaction with oxygen.
- Oxidation stateMineral chemistry
- A value describing the effective electron charge assigned to an element in a compound.
- Oxidation zoneEconomic geology
- The near-surface portion of a sulfide-bearing deposit where oxygen-rich waters transform primary minerals into secondary minerals.
- OxideMineral classes
- A mineral in which oxygen is bonded to one or more metals or semimetals without a more complex oxyanion group.
P
- ParagenesisGeologic occurrence
- The sequence and association in which minerals formed in a deposit or specimen. It helps reconstruct the history of mineralization.
- Paragenetic sequenceGeologic occurrence
- The inferred order in which minerals or mineral generations formed. Overgrowths, replacements, and inclusions can help establish the sequence.
- Parallel growthGrowth features
- Two or more crystals of the same mineral growing in nearly the same crystallographic orientation.
- ParamorphAlteration
- A pseudomorph in which one structural form of a compound changes into another polymorph while retaining the original external shape.
- PartingPhysical properties
- Breakage along structural planes caused by features such as twinning, exsolution, or deformation rather than fundamental cleavage.
- PatinaAlteration
- A surface film or alteration layer that develops with age or exposure.
- PavilionLapidary
- The portion of a faceted gemstone below the girdle.
- Peacock oreCollector terminologyTrade
- A trade and collector name usually applied to iridescent bornite or tarnished chalcopyrite. It is not a mineral species.
Also: Peacock rock
- Pearly lusterLuster
- A soft sheen resembling mother-of-pearl, often seen on cleavage surfaces or layered minerals.
- PegmatiteGeologic occurrence
- An exceptionally coarse-grained igneous rock, commonly granitic, that forms from evolved melts rich in volatile and rare elements.
- Pegmatite pocketGeologic occurrenceCollector
- A cavity within a pegmatite where late-stage fluids allowed unusually large, well-formed crystals to grow.
- PegmatiticGeologic occurrence
- Describing mineral growth, texture, or deposits associated with pegmatites or similarly coarse late-stage igneous conditions.
- Penetration twinTwinning
- A twin in which two or more crystal individuals appear to interpenetrate according to a specific symmetry relationship.
- Perfect cleavagePhysical properties
- Cleavage that produces smooth, flat surfaces very readily. Mica and calcite provide familiar examples.
- PerimorphAlteration
- A crust of one mineral that coats another and preserves its external form, even if the inner mineral later disappears.
- Petrographic microscopeOptical mineralogy
- A microscope equipped with polarizing optics for identifying minerals and textures in thin section.
Also: Polarizing microscope
- Petroleum quartzCollector terminologyCollector
- A collector term for quartz containing hydrocarbon-bearing fluid inclusions that may fluoresce under ultraviolet light.
- Phantom crystalGrowth features
- A crystal containing a visible outline of an earlier growth stage, usually marked by inclusions, color zoning, or a coating later overgrown.
- Phantom zoningGrowth featuresCollector
- An internal outline marking an earlier crystal shape or growth stage.
Also: Phantom crystal
- PhaseFundamentals
- A physically and chemically distinct material within a system. A specimen may contain several mineral phases.
- PhenocrystPetrology
- A crystal in an igneous rock noticeably larger than crystals in the surrounding groundmass.
- PhosphateMineral classes
- A mineral containing phosphate groups, commonly PO4. Apatite and turquoise are examples.
- PhosphorescenceLuminescence
- Light emission that continues for a noticeable time after the exciting source has been removed.
- PhotochromismOptical phenomena
- A reversible color change caused by exposure to light.
- PhyllosilicateMineral classes
- A silicate built from sheets of linked tetrahedra. Micas, clays, and talc belong here.
Also: Sheet silicate
- PinacoidCrystallography
- A crystal form consisting of two parallel faces related by symmetry.
- Placer depositEconomic geology
- A concentration of dense, resistant minerals produced by mechanical sorting in rivers, beaches, or other sedimentary environments.
- Plane-polarized lightOptical mineralogy
- Light vibrating mainly in one plane, used in petrographic microscopy to examine color, pleochroism, relief, and other properties.
Also: PPL
- Platelet inclusionInclusions
- A thin, plate-like included crystal or exsolved particle.
- PlatyHabit & form
- Occurring as thin, flat plates or flakes, often with less perfectly developed outlines than tabular crystals.
- Play-of-colorOptical phenomena
- Flashes or patches of changing spectral colors characteristic of precious opal, produced by diffraction from ordered silica spheres.
- PleochroismOptical properties
- A change in body color seen when an anisotropic crystal is viewed in different crystallographic directions.
- PlutonicGeologic occurrence
- Formed or emplaced at depth from magma that cools relatively slowly. Granite is a common plutonic rock.
Also: Intrusive
- PocketCollectingCollector
- A collector and mining term for a crystal-bearing cavity or localized concentration of desirable mineral specimens.
- Pocket clayCollectingCollector
- Soft clay or altered material found in some crystal pockets, especially pegmatites.
- Pocket mudCollectingCollector
- An informal term for fine material filling a crystal pocket.
- Point groupCrystallography
- The set of symmetry operations that leave at least one point fixed. The 32 crystallographic point groups correspond to the crystal classes.
- PolishLapidary
- The smoothness and finish of a worked mineral or gem surface.
- PolymorphMineral chemistry
- One of two or more minerals with the same chemical composition but different crystal structures. Diamond and graphite are polymorphs.
- Polysynthetic twinningTwinning
- Repeated twinning in many thin parallel lamellae. Plagioclase feldspar commonly shows it.
- PolytypeCrystallography
- A structural variant in which layers are stacked in different sequences while the basic layer structure remains similar.
- Poor cleavagePhysical properties
- Weakly expressed cleavage that may be difficult to distinguish from ordinary breakage.
- PorphyriticPetrology
- An igneous texture with conspicuously larger crystals, called phenocrysts, set in a finer-grained groundmass.
- PorphyryPetrology
- An igneous rock with porphyritic texture, or in mining usage a large intrusive-related deposit type. Context determines the meaning.
- Porphyry depositEconomic geology
- A large, generally low-grade mineral deposit associated with intrusive systems, widespread veins, and alteration.
- PowderyHabit & form
- Occurring as loose or easily crumbled fine particles.
- PraseCollector terminologyHistorical
- A traditional varietal term used for certain green quartz or chalcedony materials. Usage varies.
- PrasioliteCommon mineralsVarietal
- A green quartz variety, most commonly produced by heat treatment of suitable quartz, though natural examples exist.
Also: Green amethyst
- PreformLapidary
- A roughly shaped piece of gem material prepared before final cutting or polishing.
- Prepared specimenCollectingCollector
- A specimen cleaned, trimmed, stabilized, etched, or otherwise worked after collection.
- Primary mineralGeologic occurrence
- A mineral that formed during the original crystallization or formation of a rock or deposit.
- Prism formCrystallography
- A crystal form made of faces parallel to a common crystallographic direction.
- PrismaticHabit & form
- Elongated in one direction with a prism-like appearance and relatively well-developed side faces. Quartz commonly forms prismatic crystals.
- Propylitic alterationAlteration
- A hydrothermal alteration assemblage commonly involving chlorite, epidote, calcite, and albite.
- ProspectMining
- A site investigated for possible mineralization or economic potential but not necessarily developed into a mine.
- ProspectingField methods
- Searching for mineral deposits using field observations, sampling, geochemistry, geophysics, or other methods.
- ProvenanceCollecting
- The documented history of ownership, collection, and locality information associated with a specimen.
- Pseudo-octahedralHabit & form
- Having an outward shape resembling an octahedron without corresponding octahedral symmetry.
- PseudochromaticColor & optics
- Describing color produced mainly by optical effects such as interference, diffraction, or scattering rather than body-color pigments.
- PseudocubicHabit & form
- Having an external shape that resembles a cube even though the mineral is not crystallographically cubic.
- PseudohexagonalHabit & form
- Appearing six-sided because of twinning or habit even though the underlying symmetry is not hexagonal.
- PseudomorphAlteration
- A mineral or aggregate that has the external shape of another mineral because the original was replaced, altered, coated, or transformed while its form was preserved.
- PseudomorphismAlteration
- The process or condition in which one material preserves the external form of another.
- Pyramid formCrystallography
- A set of equivalent crystal faces that meet toward one end of a symmetry axis.
- PyriteCommon minerals
- Iron disulfide, FeS2, commonly forming cubes, pyritohedra, and striated crystals with metallic luster.
- Pyrite decaySpecimen care
- Deterioration of unstable pyrite or marcasite through oxidation, often producing acidic products, expansion, and crumbling.
Q
- QuarryMining
- An open excavation where stone, aggregate, industrial minerals, or other materials are extracted.
- QuartzCommon minerals
- A common crystalline mineral composed of silicon dioxide, SiO2. It occurs in many colors and habits and is a major component of many rocks and veins.
- QuartzitePetrology
- A hard metamorphic rock formed mainly from quartz-rich sandstone.
R
- RadiatingHabit & form
- Crystals growing outward from a common center or line. Radiating aggregates may form sprays, stars, or spheres.
- RadioactivityPhysical properties
- The spontaneous emission of ionizing radiation from unstable atomic nuclei. Some uranium- and thorium-bearing minerals are naturally radioactive.
- Rainbow inclusionCollector terminologyCollector
- An informal term for spectral colors produced by interference at internal fractures, thin films, or inclusions.
- Raman spectroscopyAnalytical methods
- An analytical technique that identifies materials from the characteristic way their vibrations scatter laser light.
- ReconstructionCollectingCollector
- Assembly of multiple pieces, sometimes with fill material, to recreate a more complete specimen.
- Record keeperCollector terminologyTrade
- A metaphysical and collector-market term for triangular growth marks or raised triangular features on some quartz crystals. It is not formal mineralogy.
- ReductionMineral chemistry
- A chemical process involving gain of electrons.
- RefractionOptical properties
- The change in direction of light as it passes between materials with different optical properties.
- Refractive indexOptical properties
- A measure of how strongly a material slows and bends light compared with a vacuum.
- Regional metamorphismGeologic occurrence
- Metamorphism affecting broad areas, commonly during mountain building.
- RegrowthGrowth features
- New mineral growth that develops on an older crystal or broken surface after an interruption in growth.
- Rehealed crystalCollectingCollector
- A crystal whose broken surface was partly or completely covered by later natural crystal growth.
Also: Self-healed crystal
- ReliefOptical mineralogy
- The apparent degree to which a mineral grain stands out from its surroundings in thin section.
- ReniformHabit & form
- Kidney-shaped, with rounded lobes generally larger and less grape-like than botryoidal forms.
- RepairCollectingCollector
- A specimen that has been reattached with adhesive after breaking. Responsible sale and cataloging normally disclose repairs.
- Repaired specimenCollectingCollector
- A specimen whose broken pieces have been reattached, usually with adhesive.
- Repeated twinningTwinning
- A general term for crystals composed of more than two twin-related individuals.
- ReplacementAlteration
- A process in which one mineral dissolves while another forms in its place, often preserving part or all of the original texture or shape.
- Replacement depositEconomic geology
- A mineral deposit formed largely by chemical replacement of pre-existing rock.
- Residual depositEconomic geology
- A concentration of minerals left behind after weathering removes more soluble or mobile components.
- Resinous lusterLuster
- A surface appearance resembling resin or amber. Sphalerite may show a resinous to adamantine luster.
- Respirable dustSafety
- Very fine airborne particles small enough to penetrate deep into the lungs.
- RestorationCollectingCollector
- Addition or reconstruction of missing material to improve a specimen’s appearance. Restoration should be disclosed.
- ReticulatedHabit & form
- Forming a network or lattice-like pattern of intersecting crystals or branches.
- Reverse scepterHabit & formCollector
- A scepter-like crystal in which the later-grown upper part is narrower than the underlying section.
- RhodochrositeCommon minerals
- Manganese carbonate, MnCO3, known for pink to red rhombohedral crystals and banded masses.
- Rhombohedral cleavagePhysical properties
- Cleavage in three directions that do not meet at right angles, producing rhombohedron-shaped fragments. Calcite is the classic example.
- RhombohedronCrystallography
- A six-faced crystal form whose faces are rhombi; opposite faces are parallel. Calcite commonly displays it.
- RindGrowth features
- An outer layer on a mineral or rock that differs from the interior because of weathering, alteration, growth, or deposition.
- RoadcutField methods
- A man-made rock exposure created when a road is cut through bedrock or sediment.
- RockFundamentals
- A naturally occurring solid aggregate of one or more minerals, mineraloids, glass, or organic material. Granite, basalt, and limestone are rocks rather than individual minerals.
- Rock crystalCommon mineralsVarietal
- A traditional name for transparent, colorless quartz.
Also: Clear quartz
- Rock-forming mineralPetrology
- A mineral that commonly makes up a significant part of rocks. Feldspars, quartz, pyroxenes, amphiboles, micas, and olivine are major examples.
- Rose cutLapidary
- A historical faceting style with a flat base and domed top made of triangular facets.
- Rose quartzCommon mineralsVarietal
- A pink variety of quartz. Most massive rose quartz owes its color to microscopic inclusions.
- RosetteHabit & form
- An aggregate of platy or bladed crystals arranged like the petals of a rose.
- RoughLapidary
- Uncut or minimally processed gem or lapidary material.
- Rounded crystalGrowth features
- A crystal whose edges and faces have been softened by dissolution, abrasion, or growth effects.
- Rutilated quartzCollector terminologyVarietal
- Quartz containing visible needle-like inclusions of rutile. It is a descriptive variety, not a separate mineral species.
- RutileCommon minerals
- Titanium dioxide, TiO2, commonly forming prismatic or acicular crystals. Fine rutile needles are important gem inclusions.
S
- Satin sparCommon mineralsVarietal
- A fibrous variety of gypsum with a silky sheen. The name has also been used historically for fibrous calcite.
- ScalenohedronCrystallography
- A closed crystal form with twelve scalene triangular faces. Calcite commonly forms scalenohedra.
- Scanning electron microscopeAnalytical methods
- An instrument that scans a sample with electrons to produce high-resolution images and, with detectors, chemical information.
Also: SEM
- Scepter crystalHabit & form
- A crystal with a wider or differently shaped termination grown on top of a narrower stem, producing a scepter-like outline.
- SchillerOptical phenomena
- A metallic or pearly internal sheen produced by oriented inclusions, exsolution layers, or structural features.
- SchistPetrology
- A metamorphic rock with strong foliation and abundant visible platy or elongated minerals.
- ScintillationGemology
- The pattern of flashes seen as a faceted gem, light source, or observer moves.
- Secondary mineralGeologic occurrence
- A mineral formed by alteration, weathering, oxidation, or later reaction from pre-existing minerals.
- SectilePhysical properties
- Capable of being cut smoothly with a knife, producing shavings rather than powder.
- Sector zoningGrowth features
- Chemical or color differences between sectors of a crystal that grew on different faces at the same time.
- SedimentaryGeologic occurrence
- Related to sediments and rocks formed by deposition, precipitation, or biological accumulation near Earth’s surface.
- Sedimentary rockPetrology
- Rock formed by deposition and lithification of sediment, chemical precipitation, or accumulation of biological material.
- SeleniteCommon mineralsVarietal
- A traditional varietal name for clear or transparent crystalline gypsum. It is gypsum, not a separate mineral species.
- Self-healed crystalCollectingCollector
- A crystal that was broken or detached and later developed new crystal faces over the damaged surface during renewed growth.
Also: Rehealed crystal
- SEMAnalytical methods
- Abbreviation for scanning electron microscopy or scanning electron microscope.
Also: Scanning electron microscope
- SericitizationAlteration
- Hydrothermal alteration that produces fine-grained white mica, commonly from feldspars.
- SerpentinitePetrology
- A rock composed largely of serpentine-group minerals, formed mainly by alteration of ultramafic rocks.
- SerpentinizationAlteration
- Hydration and alteration of magnesium-rich rocks, especially ultramafic rocks, to serpentine-group minerals.
- ShaftMining
- A steep or vertical underground mine opening used for access, hoisting, ventilation, or services.
- Sharp crystalCollectingCollector
- An informal collector description for a crystal with crisp, well-defined faces, edges, and termination.
- SheaflikeHabit & form
- An aggregate in which elongated crystals fan outward like a bundle of grain stalks.
- Short-prismaticHabit & form
- Prismatic crystals that are only moderately longer than they are wide.
- Shortwave UVLuminescence
- Shorter-wavelength ultraviolet radiation commonly around 254 nm in traditional mineral lamps. Eye and skin protection are important.
- Show specimenCollectingCollector
- An informal term for an attractive specimen selected for display at a mineral show or in a showcase.
- SilicaMineral chemistry
- Silicon dioxide, SiO2, occurring in crystalline minerals such as quartz and in amorphous or poorly crystalline materials such as opal.
- Silica dustSafety
- Airborne particles containing crystalline silica, especially from cutting, grinding, or crushing quartz-rich material.
- SilicateMineral classes
- A mineral containing silicon-oxygen structural units, especially SiO4 tetrahedra. Silicates make up most of Earth’s crust and mantle.
- SilicificationAlteration
- Addition or replacement by silica, commonly producing quartz-rich rock or mineralization.
- Silky lusterLuster
- A sheen produced by parallel fine fibers, giving the surface a silk-like appearance.
- SimulantTreatments
- A material used to imitate the appearance of another gem or mineral while differing in composition, structure, or both.
Also: Imitation
- Single-terminatedCollectingCollector
- Having a natural termination at one end while the other end was attached, broken, or embedded.
- SkarnGeologic occurrence
- A rock and mineral assemblage formed by reaction between an intrusion or hydrothermal fluids and reactive rocks, commonly limestone or dolostone.
- Skeletal crystalHabit & form
- A crystal with incomplete, open, or frame-like growth caused by rapid growth under non-equilibrium conditions.
- Skeletal growthHabit & form
- Crystal growth dominated by edges, corners, or branching frameworks rather than complete filled faces.
- SlabLapidary
- A flat slice of rock or mineral cut with a saw.
- Small cabinet specimenCollectingCollector
- A collector-size category between miniature and full cabinet size. Exact limits vary.
- SmithsoniteCommon minerals
- Zinc carbonate, ZnCO3, a secondary zinc mineral that commonly forms colorful botryoidal aggregates.
- Smoky quartzCommon mineralsVarietal
- A brown to gray variety of quartz whose color is associated with irradiation-related color centers.
- Solid inclusionInclusions
- A crystal or solid particle enclosed within another mineral.
- Solid solutionMineral chemistry
- A crystalline phase in which one element or ion can substitute for another within structural limits while the basic structure remains the same.
- Soluble mineralSpecimen care
- A mineral that dissolves relatively readily in water or another common solvent.
- SorosilicateMineral classes
- A silicate built mainly from paired Si2O7 groups. Epidote-group minerals are examples.
- Space groupCrystallography
- A complete description of crystal symmetry including point symmetry and translational symmetry. There are 230 three-dimensional crystallographic space groups.
- Specific gravityPhysical properties
- The ratio of a material’s density to the density of water, giving a unitless comparison of how heavy it is for its size.
- SpecimenCollecting
- A piece of mineral, rock, ore, or other geologic material collected for study, reference, teaching, or display.
- Specimen conservationSpecimen care
- The practice of preserving mineral specimens and labels while minimizing deterioration and unnecessary treatment.
- SpeleothemGeologic occurrence
- A secondary mineral deposit formed in a cave, commonly from calcite or aragonite.
- SphaleriteCommon minerals
- Zinc sulfide, ZnS, the principal zinc ore mineral, commonly with resinous to adamantine luster.
- SpheruliticHabit & form
- Made of roughly spherical bodies formed by radiating crystals or fibers.
- Spirit quartzHabit & formTrade
- A trade name, especially for South African cactus quartz, in which a central quartz crystal is coated by many smaller quartz crystals.
Also: Cactus quartz, Porcupine quartz
- Splintery fracturePhysical properties
- Breakage into elongated, splinter-like fragments.
- SpodumeneCommon minerals
- A lithium aluminum pyroxene occurring in pegmatites and known for gem varieties kunzite and hiddenite.
- SprayHabit & formCollector
- A cluster of slender crystals spreading from a common base, often in a fan or radiating arrangement.
- StabilizedCollectingCollector
- Treated with resin, glue, or another material to strengthen fragile mineral or matrix.
- StalactiticHabit & form
- Growing as hanging or projecting cylindrical forms resembling cave stalactites, often with concentric internal layers.
- StauroliteCommon minerals
- An iron-aluminum silicate mineral common in metamorphic rocks and famous for cross-shaped penetration twins.
- StellateHabit & form
- Arranged in a star-like cluster, commonly with crystals radiating from a central point.
- Step cutLapidary
- A faceting style using long, usually rectangular facets arranged in steps.
- StockworkEconomic geology
- A dense network of intersecting mineralized veinlets rather than one dominant vein.
- StopeMining
- An underground excavation from which ore has been removed.
- StreakPhysical properties
- The color of a mineral in powdered form, usually observed by rubbing it on unglazed porcelain.
- Streak plateField methods
- A piece of unglazed porcelain used to test mineral streak. Very hard minerals may scratch it rather than leave useful powder.
- Subconchoidal fracturePhysical properties
- A fracture that is partly shell-like but less smoothly curved than true conchoidal fracture.
- SubhedralHabit & form
- Having some recognizable crystal faces but not a completely well-formed external shape.
- SublimateGeologic occurrence
- A mineral deposited directly from vapor or gas without first becoming liquid.
- Submetallic lusterLuster
- A luster that is somewhat metallic but less bright or reflective than a true metallic luster.
- Subparallel growthGrowth features
- An aggregate of crystals with similar but not perfectly identical orientations.
- SubstitutionMineral chemistry
- Replacement of one ion by another in a crystal structure. Similar size and charge often make substitution easier.
- SulfateMineral classes
- A mineral containing sulfate, SO4, as an essential structural component. Gypsum and barite are examples.
- SulfideMineral classes
- A mineral in which sulfur is bonded mainly with metals or semimetals in a reduced state. Pyrite, galena, and chalcopyrite are examples.
- SulfosaltMineral classes
- A complex sulfide mineral containing a semimetal such as arsenic, antimony, or bismuth together with metals and sulfur.
- SupergeneEconomic geology
- Formed or modified near Earth’s surface by downward-moving waters, oxidation, and weathering processes.
- SupergroupClassification
- A broad formal grouping of structurally related mineral groups used in some modern classifications.
- SymmetryCrystallography
- The ordered repetition of equivalent features in a crystal.
- Symmetry elementCrystallography
- A geometric feature such as an axis, plane, or point associated with a symmetry operation.
- Symmetry operationCrystallography
- A movement or transformation that brings a crystal structure into an indistinguishable configuration.
- SynonymNomenclature
- An alternative name that refers to the same mineral species or concept. Scientific references usually prefer one accepted name.
- Syntaxial intergrowthCrystallography
- An oriented intergrowth in which related crystal structures share closely corresponding directions or planes.
- Synthetic mineralTreatments
- A man-made crystalline material with essentially the same composition and crystal structure as a natural mineral counterpart.
T
- Table facetLapidary
- The large, usually topmost central facet of many faceted gemstones.
- TabularHabit & form
- Flattened into broad, plate-like crystals with one dimension much smaller than the other two. Barite often forms tabular crystals.
- TailingsMining
- Finely ground waste material left after valuable minerals have been separated from ore.
- TarnishAlteration
- A thin surface alteration film produced by chemical reaction. Tarnish can create iridescent colors on bornite and chalcopyrite.
- TasteField methodsHistorical
- A historical identification property once used for soluble minerals such as halite. Tasting unknown minerals is not recommended.
- TectosilicateMineral classes
- A framework silicate in which tetrahedra are linked in a three-dimensional network. Quartz and feldspars are examples.
Also: Framework silicate
- TenacityPhysical properties
- How a mineral responds to bending, cutting, hammering, or pulling rather than scratching.
- TenebrescenceOptical phenomena
- A reversible color change triggered by light, especially ultraviolet, followed by fading back toward the original color. Hackmanite is well known for it.
Also: Reversible photochromism
- Terminated crystalCollectingCollector
- A crystal whose end is naturally closed by crystal faces rather than broken off.
- TerminationHabit & form
- The set of faces closing the end of a crystal.
- Tetragonal systemCrystallography
- A crystal system with three perpendicular axes, two equal in length and a third of different length.
- TetrahedronCrystallography
- A four-faced form composed of triangular faces.
- TexturePetrology
- The size, shape, arrangement, and relationships of grains or crystals in a rock or mineral aggregate.
- ThermoluminescenceLuminescence
- Light emitted when a material is heated after energy has previously been stored in crystal defects.
- Thin sectionOptical mineralogy
- A very thin slice of rock or mineral mounted on glass so transmitted light can pass through it for microscopic study.
- Three-phase inclusionInclusions
- An inclusion containing three distinguishable phases, such as liquid, vapor, and a solid daughter crystal.
- Thumbnail specimenCollectingCollector
- A small mineral specimen conventionally sized to fit in a small display box; collector conventions vary.
- TopazCommon minerals
- An aluminum fluorosilicate known for high hardness, perfect basal cleavage, and gem-quality crystals.
- Tourmalinated quartzCollector terminologyVarietal
- Quartz containing visible tourmaline inclusions, commonly black schorl needles or prisms.
- TourmalineCommon minerals
- A complex group of boron-bearing silicate minerals known for prismatic crystals, strong color zoning, and diverse compositions.
- Toxic mineralSafety
- A mineral containing elements or compounds that can be harmful if inhaled, ingested, or otherwise introduced into the body.
- Trace elementMineral chemistry
- An element present in very small concentration that may strongly influence color, fluorescence, or other properties.
- Trade nameNomenclatureTrade
- A commercial name used to market a mineral, gem, rock, or variety. A trade name does not necessarily correspond to a formal mineral species.
- TranslucentPhysical properties
- Allowing some light to pass through but scattering it enough that objects are not seen clearly.
- TransparencyPhysical properties
- The degree to which light passes through a mineral. Common descriptions are transparent, translucent, and opaque.
- TransparentPhysical properties
- Allowing light to pass through so objects can be seen clearly through the material.
- TrapezohedronCrystallography
- A closed form with kite-shaped faces. Garnets commonly show forms traditionally called trapezohedra by collectors.
- TrenchingField methods
- Excavating shallow trenches to expose bedrock, veins, or mineralized zones.
- TriboluminescenceLuminescence
- Light produced when certain materials are rubbed, fractured, scratched, or crushed.
- TrichroismOptical properties
- Pleochroism showing three principal colors in different directions in a biaxial crystal.
- Triclinic systemCrystallography
- The crystal system with the least symmetry, conventionally described by three unequal axes with no angles constrained to 90 degrees.
- Trigonal systemCrystallography
- A crystal system characterized by a threefold principal symmetry axis.
- Trimmed specimenCollectingCollector
- A specimen whose matrix or unwanted portions have been cut or broken away to improve size, shape, stability, or presentation.
- TrimorphMineral chemistry
- One of three minerals sharing the same chemical composition but having different crystal structures.
- TumblingLapidary
- Smoothing and polishing stones by rotating or vibrating them with abrasive grit and polishing media.
- TungstateMineral classes
- A mineral containing tungstate groups, commonly WO4. Scheelite is an example.
- Twin axisTwinning
- An axis about which rotation relates one individual of a twin to another.
- Twin lawTwinning
- The crystallographic rule defining the orientation relationship between individuals in a twin.
- Twin planeTwinning
- A plane that acts as a symmetry relationship between parts of certain twinned crystals.
- TwinningTwinning
- The regular, symmetry-related intergrowth of two or more crystals of the same mineral.
- Two-phase inclusionInclusions
- A fluid inclusion containing two visible phases, commonly liquid plus a gas bubble.
- Type localityClassification
- The locality from which the material used to define a new mineral species was obtained.
- Type specimenClassification
- The reference specimen or material associated with the formal description of a mineral species and deposited in a recognized collection.
U
- Ultrasonic cleanedSpecimen careCollector
- Cleaned using high-frequency vibrations in a liquid bath. This can damage fragile, fractured, or porous minerals.
- Underground mineMining
- A mine developed below the surface through shafts, adits, declines, tunnels, and workings.
- Uneven fracturePhysical properties
- Irregular breakage that produces rough, non-planar surfaces.
- UniaxialOptical properties
- Describing an anisotropic crystal with one optic axis. Tetragonal, trigonal, and hexagonal crystals are uniaxial.
- Unit cellCrystallography
- The smallest repeating three-dimensional building block that expresses the translational symmetry of a crystal structure.
- UV fluorescenceLuminescenceCollector
- Fluorescence produced by ultraviolet radiation. Collectors commonly compare longwave and shortwave responses.
Also: Ultraviolet fluorescence
V
- ValenceMineral chemistry
- A measure of an atom’s combining capacity, closely related to bonding and oxidation state.
- VanadateMineral classes
- A mineral containing vanadate groups, commonly VO4. Vanadinite is an example.
- Varietal nameNomenclature
- A name for a recognizable variety of a mineral rather than a separate species. Citrine and amethyst are varietal names for quartz.
- Veil inclusionInclusionsCollector
- A sheet-like plane of tiny fluid inclusions or healed fractures that looks like a veil inside a transparent crystal.
- VeinGeologic occurrence
- A tabular or branching mineral-filled fracture that cuts through rock.
- VeinletGeologic occurrence
- A small or thin mineral-filled fracture. Networks of veinlets can produce stockwork mineralization.
- VesiclePetrology
- A cavity in volcanic rock formed by a gas bubble in magma or lava.
- VesicularPetrology
- Containing many vesicles or gas-bubble cavities. Basalt can be strongly vesicular.
- Vitreous lusterLuster
- A glass-like luster and one of the most common nonmetallic lusters. Quartz commonly appears vitreous.
Also: Glassy luster
- VolcanicGeologic occurrence
- Related to magma erupted at or near Earth’s surface. Volcanic rocks often contain cavities later filled by secondary minerals.
Also: Extrusive
- VugGeologic occurrence
- A small to moderate cavity in rock, often lined with crystals that grew into open space.
Also: Vugh
W
- Waste rockMining
- Rock removed during mining that is not processed as ore.
- Water-clearCollectingCollector
- An informal collector and gem term for exceptionally transparent, colorless material with little visible cloudiness.
- Waxy lusterLuster
- A subdued sheen resembling wax. Chalcedony commonly has a waxy luster.
- WeatheringGeologic occurrence
- Physical and chemical breakdown or alteration of minerals and rocks at or near Earth’s surface.
- Wheat-sheaf habitHabit & form
- A bundle-like aggregate of elongated crystals that narrow near the center and flare outward at the ends.
- Window crystalCollector terminologyCollector
- An informal collector term for crystals showing a flat or recessed face that appears window-like. It has no single formal mineralogical definition.
- WiryHabit & formCollector
- Growing in thin, twisting, wire-like forms. Native copper and silver may show wiry growth.
X
- X-ray diffractionAnalytical methods
- An analytical technique that identifies crystalline materials by their characteristic X-ray diffraction pattern.
Also: XRD
- X-ray fluorescenceAnalytical methods
- An analytical method that determines elemental composition by measuring characteristic X-rays emitted from a sample.
Also: XRF
- XRDAnalytical methods
- Abbreviation for X-ray diffraction, one of the most widely used methods for identifying crystalline mineral phases.
Also: X-ray diffraction
- XRFAnalytical methods
- Abbreviation for X-ray fluorescence, a technique used to measure elemental composition.
Also: X-ray fluorescence
Z
- ZoneCrystallography
- A set of crystal faces whose intersection lines are parallel to a common crystallographic direction.
- Zone axisCrystallography
- The common direction shared by faces belonging to a crystallographic zone.