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Mineraloid · Mineraloid · Volcanic Glass

Obsidian

SiO₂-rich volcanic glass

Nature's glass — lava quenched so fast it never had time to crystallize.

Hardness5–5.5
Crystal systemAmorphous
LusterVitreous
Sp. gravity2.33–2.42
ClassMineraloid
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Obsidian specimen

Obsidian — Photo: James St. John · CC BY 2.0

What is Obsidian?

Obsidian is volcanic glass — silica-rich lava that cooled too fast for crystals to organize. Chemically it is roughly rhyolite; structurally it is not a mineral at all but a mineraloid, an amorphous solid.

Its conchoidal fracture produces edges a few molecules wide — sharper than any metal. That made obsidian the steel of the Stone Age, traded over thousands of kilometers; archaeologists fingerprint its trace elements to map ancient trade routes.

In one sentence: Black, glassy, sharp-edged — the mineraloid that built prehistory's toolkit.

Physical Properties

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

Fracture. Conchoidal flakes with edges under 3 nm — far keener than scalpels.

Varieties. Snowflake obsidian carries cristobalite spherulites; sheen types show magnetite or gas-void layers.

Dating. Fresh obsidian slowly hydrates — the hydration rind's thickness estimates age (obsidian hydration dating).

What It’s Used For

Lithic toolsConchoidal flake edges rival surgical steel — used for blades for 1.5 million years.
LapidarySnowflake and rainbow varieties cut popular cabochons.
ScalpelsModern obsidian scalpels make finer edges than steel.

How to Identify It

Glassy conchoidal fracture — sharp flakesBlack glass with white streakNo crystals, no cleavageLighter than basalt (SG ~2.4)
Looks like…But differs because…Quick test
JetOrganic lignite — lighter, warm to the touch, brown streakStreak & feel
FlintCryptocrystalline, duller luster, not glassyLuster

Where It’s Found

Notable localities for fine specimens:

Obsidian Cliff, United States Glass Buttes, United States Lipari, Italy Pachuca, MexicoOpen the locality map →