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Silicate · Amorphous hydrated silica (mineraloid)

Opal

SiO₂·nH₂O

Hydrated silica stacked in microscopic spheres — a mineraloid whose ordered inner layers split light into the famous play-of-color.

Hardness5.5–6.5
Crystal systemAmorphous
LusterVitreous
Sp. gravity1.9–2.3
ClassSilicate
←PrevOosterboschiteNextOphirite→
Opal specimen

Precious opal — silica spheres ~400 nm across diffract light into play-of-color — Photo: James St. John · CC BY 2.0

What is Opal?

Opal is hydrated silica — SiO₂·nH₂O — that never managed to crystallize. Under the electron microscope it's a stack of silica spheres tens to hundreds of nanometers across, glued together with water making up roughly 3–21% of its weight. Because it has no ordered crystal structure, opal is technically a mineraloid, kept on the mineral books for historical reasons.

Its magic trick is play-of-color. In precious opal the spheres are nearly identical in size and settle into orderly layers — a natural diffraction grating. Light waves bouncing off successive layers interfere, splitting white light into flashes of pure spectral color that roll across the stone as it turns. Random sphere sizes produce common opal — pretty but colorless of fire.

The family is richer than the rainbow suggests: fire opal is transparent orange-to-red body color without play-of-color; black opal puts the fire against a dark body and commands the highest prices; hydrophane opal from Ethiopia is porous enough to soak up water and go transparent; glassy hyalite beads up on rock surfaces like frozen water — which, nearly, it is.

In one sentence: Opal is softer and lighter than quartz (H 5.5–6.5, SG ~2), amorphous, and contains water — which is why it can craze or crack if it dries out or heats suddenly.

Physical Properties

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

Hardness & handling. At Mohs 5.5–6.5 with a brittle conchoidal fracture, opal is a gemstone that demands care. Its water content is the real quirk: sudden drying or heat can trigger crazing — a web of internal cracks — so miners sometimes store rough in water.

Density & optics. Specific gravity runs about 1.9–2.3 — notably light — and refractive index a low 1.37–1.52. As an amorphous solid it's isotropic: no pleochroism, no birefringence, just the play-of-color.

Fluorescence. Common opal often glows greenish-white under UV, and some material phosphoresces after the light goes off — a useful field clue.

How & Where It Forms

Most precious opal is a story of weathering: silica leached from rocks by water percolates down into seams and voids, then dries into gel that hardens over millennia — Australia's opal fields formed this way in an ancient inland sea's sediments. Mexican fire opal instead fills gas cavities in rhyolite lava. Opal also famously replaces buried wood, bone and shell, preserving the original's shape atom-less but form-perfect.

Often grows alongside:

QuartzChalcedonyTridymiteCristobaliteCalcite

What It’s Used For

GemstonesOctober birthstone. Black, boulder, crystal, white and fire opals are cut as cabochons or faceted gems.
Ornamental carvingsCommon opal in pink, blue and green is carved into beads and objects.
Collector specimensHyalite crusts, opalized fossils and play-of-color seam specimens.

How to Identify It

Look for play-of-color that moves — real spectral patches, not surface iridescence. It's lighter and softer than quartz or chalcedony, breaks in smooth curved shells, and warmth on the lip (the old 'lip test') hints at its water content. Beware 'opalite' glass and assembled doublets/triplets where a thin opal slice rides on dark backing.

Play-of-color — patches of spectral color that shift as the stone turnsSofter and lighter than quartz: H 5.5–6.5, SG ~1.9–2.3Conchoidal fracture, waxy-to-vitreous luster, no crystalsContains water — can craze (crack) if dried or heated
Looks like…But differs because…Quick test
ChalcedonyMicrocrystalline quartz is harder (~7), denser (SG 2.6) and never shows true play-of-colorHeft & hardness
MoonstoneAdularescence is a single billowy sheen from feldspar layers, not shifting spectral patches; moonstone has cleavageWatch the light move
LabradoriteLabradorescence flashes in broad sheets along cleavage planes; a feldspar with H 6–6.5 and twinningCleavage & flash pattern
QuartzHarder (7), denser (2.65), forms crystals — quartz lacks play-of-color and water contentHardness & SG

Where It’s Found

Notable localities for fine specimens:

Lightning Ridge, Australia Coober Pedy, Australia Andamooka, Australia Querétaro, Mexico Wegel Tena, Wollo, Ethiopia Virgin Valley, USAOpen the locality map →

References & Further Reading

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

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