Polished black obsidian

Hardness about 5–5.5 Mohs; opaque black to dark gray, vitreous luster, conchoidal fracture, and no crystal structure or cleavage. The surface appears polished and may show flow lines or natural cracks.

Rapid cooling of silica-rich lava prevented crystallization, usually during Quaternary volcanic activity. Commonly associated with rhyolitic lava domes and flows.

Used for ornaments, cabochons, beads, carvings, and historically razor-sharp tools and arrowheads. Common polished pieces have modest collecting value.

Obsidian can form exceptionally sharp edges because it fractures conchoidally; microscopic crystals may eventually develop as the glass devitrifies.

Identify by its glassy luster, uniform dark color, light weight relative to many black stones, and curved shell-like fracture; it scratches glass but is scratched by harder minerals. Common localities include Mexico, western North America, Iceland, Armenia, and Japan.