Obsidian

Obsidian is a natural volcanic glass, usually black and rich in silica, that forms when lava cools so fast that crystals cannot grow. It breaks along smooth, curved (conchoidal) surfaces with extremely sharp edges, and for thousands of years it was a prime material for blades and arrowheads.

🪨 Geology Guide Specimen: Open Obsidian in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.

When viscous, silica-rich lava is chilled quickly, as at the margin of a dome or flow, its atoms are locked in a disordered arrangement. Obsidian also holds very little water, usually under about one percent, which separates it from froth-forming pumice. Because glass is unstable over geological time, it slowly devitrifies into tiny crystals, so obsidian older than a few tens of millions of years is rare.

Its dark color comes from iron-bearing microscopic crystals and nanoparticles, not from a different composition from pale rhyolite. Hematite gives mahogany tones, cristobalite spherulites give "snowflake" obsidian, and tiny gas bubbles or oriented inclusions create gold and rainbow sheens.

Freshly broken obsidian has edges far finer than a steel blade, so it was knapped into knives, spearpoints and mirrors on every inhabited continent. Because each flow has a distinct chemical fingerprint, archaeologists trace obsidian artifacts back to their source and map ancient trade routes, and the rate at which a fresh surface absorbs water can be used to estimate when it was cut.

• Rhyolite — the crystalline rock with the same chemistry
• Pumice — the gas-rich glass that floats
• Volcanic Bomb — another product of quenched lava