Obsidian

Obsidian isn't technically a mineral. It's a volcanic glass. It forms when felsic lava (rich in silica) erupts and cools so rapidly that atoms don't have time to arrange into a crystal lattice. The result is a solid with the chemical composition of granite but the atomic structure of a liquid, frozen in an instant.

This rapid cooling typically happens when lava contacts water, air, or the edge of a lava flow. The entire process can take hours to days, vanishingly fast in geological terms. Because obsidian is metastable, it slowly devitrifies over millions of years, meaning ancient obsidian gradually transforms from glass into tiny crystals. This is why obsidian older than about 20 million years is extremely rare.

Varieties like snowflake obsidian contain cristobalite inclusions, small, radial crystal formations that formed as the glass began its slow transformation. Rainbow obsidian gets its iridescent sheen from nanoscale layers of magnetite crystals aligned within the glass. Mahogany obsidian contains iron oxide inclusions that create warm brown streaks.

Obsidian is distinctive: glassy luster, conchoidal fracture, and typically jet black. It fractures into curved, razor-sharp edges. Prehistoric peoples exploited this property to create blades sharper than modern surgical steel. At 5-5.5 on the Mohs scale, it's softer than quartz and can be scratched by a steel file.

Distinguish it from black tourmaline (which has a matte, striated surface), black onyx (banded chalcedony, waxy luster), and jet (organic, much lighter weight). Obsidian's sharp, glassy fracture surfaces are its most reliable diagnostic feature. Hold it up to strong light. Thin edges of genuine obsidian will be slightly translucent, usually with a brown or gray tint.