Black obsidian

Hardness about 5–5.5 Mohs; jet-black, vitreous luster, noncrystalline glassy structure, and no cleavage. It commonly breaks with sharp conchoidal fracture and may show flow banding or translucent edges.

Forms when silica-rich lava cools so rapidly that crystals cannot develop, typically during Quaternary volcanic eruptions. It is common around young rhyolitic volcanoes and lava domes.

Used for ornamental objects, cabochons, carvings, tools, and historically extremely sharp blades; attractive sheen varieties are collected and occasionally used in jewelry.

Its fracture can produce edges sharper than surgical steel, though they are brittle. Snowflake, rainbow, sheen, and mahogany obsidian are recognized varieties caused by inclusions or optical effects.

Identify it by its glassy black appearance, light weight relative to metallic minerals, sharp conchoidal fracture, and lack of visible crystals; distinguish it from slag by natural flow texture and uniform glass. Major sources include Mexico, the western United States, Armenia, Turkey, and Japan.