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Variety · Silicate · Tectosilicate (framework silicate) · Quartz Group · of Quartz

Agate

SiO₂

The banded quartz — concentric rings of microcrystalline silica built up inside hollow rocks, one layer at a time.

Hardness6.5–7
Crystal systemTrigonal
LusterVitreous
Sp. gravity2.58–2.64
ClassSilicate
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Agate specimen

A sliced agate nodule — concentric bands of microcrystalline quartz — Photo: Marie-Lan Taÿ Pamart · CC BY 4.0

What is Agate?

Agate is banded chalcedony — quartz grown as microscopic fibers instead of visible crystals, deposited in concentric layers inside hollows in rock. Slice one open and you read its growth history: bands of white, gray, blue, brown and red recording each change in the silica-bearing fluids that fed it.

The recipe is ancient. Gas bubbles trapped in cooling lava become hollow pockets; groundwater carrying dissolved silica seeps in and lines the walls, layer upon layer, until the pocket fills or the supply stops. Iron oxides and other impurities stain individual bands, and any last void often hosts a flourish of sparkling quartz crystals.

Named by Theophrastus for the Achates River in Sicily around 300 BC, agate has been worked since prehistory and built the cutting industry of Idar-Oberstein in Germany, where it has been carved continuously for centuries.

In one sentence: Agate is quartz, just microscopic: same chemistry (SiO₂), near the same hardness, no cleavage — but grown as a stone of layers, and usually translucent rather than clear.

Physical Properties

The full data table is in the panel alongside. Here’s what those numbers actually mean.

Texture. Under a microscope, agate is a felt of interlocking quartz fibers only thousandths of a millimeter wide — microcrystalline (‘cryptocrystalline’) quartz, often intergrown with the related silica mineral moganite. That fibrous build gives agate its slightly lower density and waxy fracture compared with coarse quartz.

Banding. The bands follow the shape of the cavity: wall-hugging, curved ‘fortification’ banding in most nodules, or straight parallel layers in seam agates. True onyx is simply agate with straight, sharply alternating bands.

Color. Natural band colors run white, gray, brown, red and soft blue. The screaming pinks and teals in gift shops are dyed — agate's porous bands take pigment readily, a trade practice going back to Roman times.

Varieties

Same chemistry, different faces — trace impurities and how finely it crystallizes give a whole family of familiar stones.

Moss Agate
Moss Agate
Clear to white with green dendrites · Manganese & iron oxide inclusions
Onyx
Onyx
Straight black-and-white bands · Evenly layered chalcedony
Sardonyx
Sardonyx
Straight red-brown and white bands · Iron-stained layers

How & Where It Forms

Most agate is born in basalt. Vast lava fields — Rio Grande do Sul in Brazil, Artigas in Uruguay, the Deccan Traps of India — cooled with billions of gas cavities. Silica-rich groundwater, heated and mineral-laden, entered the pockets and precipitated chalcedony in rhythmic layers over thousands to millions of years.

Agate also fills cracks in weathered rock, grows in sedimentary nodules, and replaces fossils and wood cell by cell. Erosion frees the nodules — tougher than their host rock — which is why famous stones like Montana's moss agate and Minnesota's Lake Superior agate are collected from riverbeds and beaches.

Often grows alongside:

CalciteZeoliteChalcedonyAmethystOpal

What It’s Used For

Gemstones & beadsCabochons, beads and tumbled stones — banded slices are a jewelry staple.
Cameos & intagliosLayered bands let carvers cut light figures against dark grounds, a craft continuous since antiquity.
Laboratory wareAgate mortars, pestles and balance knife-edges exploit its hardness and chemical inertness.
Decorative objectsSliced nodules become bookends, coasters and clock faces.

How to Identify It

Cut or break it: concentric banding plus translucency at the edges is agate. It scratches glass (hardness ~7), shows no cleavage, and fractures with a waxy conchoidal surface.

Jasper is the closest lookalike — but jasper is opaque; hold it to the light. Banded calcite sold as ‘onyx-marble’ is far softer and fizzes in dilute acid; opal is softer still and shows play-of-color rather than banding.

Concentric banding visible on a cut faceTranslucent at the edges — hold it to the lightAbout hardness 7, no cleavageWaxy-to-glassy luster on fracture surfaces
Looks like…But differs because…Quick test
JasperJasper is opaque; agate lets light through at its edgesBacklight it
CalciteBanded ‘onyx-marble’ calcite is much softer (3) and fizzes in dilute acidAcid & scratch
OpalOpal is softer (5.5–6.5), lighter and shows play-of-color rather than bandingHardness

Where It’s Found

Notable localities for fine specimens:

Idar-Oberstein, Germany Rio Grande do Sul, Brazil Artigas, Uruguay Chihuahua agate fields, Mexico Lake Superior shores, USA Bobonong district, BotswanaOpen the locality map →

References & Further Reading

  1. Dana’s New Mineralogy, 8th ed. Silica group, tectosilicates.
  2. Mindat.org — Agate mineral data and locality records.
  3. Wikipedia — Agate.

Last reviewed 2026-09-14 · How to cite this page.