igneous
Obsidian (likely black volcanic glass)
Obsidian; naturally occurring rhyolitic glass, primarily SiO₂
AI-generated identification · may be incorrectUsually black, glossy to vitreous, with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, poor cleavage, specific gravity roughly 2.35–2.과. The specimen appears dark, shiny, and irregularly fractured, though the photo cannot exclude slag or another glassy rock.
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Physical properties
Usually black, glossy to vitreous, with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, poor cleavage, specific gravity roughly 2.35–2.과. The specimen appears dark, shiny, and irregularly fractured, though the photo cannot exclude slag or another glassy rock.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in volcanic settings younger than a few million years. Commonly associated with rhyolitic lava flows and domes.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads, and today for ornaments, cabochons, carvings, and collectibles; rarely used in construction.
Geological facts
Fresh edges can be sharper than surgical steel, but obsidian is brittle and easily chipped. Snowflake, mahogany, rainbow, and sheen varieties result from inclusions or optical effects.
Field identification & locations
Look for a glassy luster, curved conchoidal breaks, sharp edges, and lack of crystal grains; test cautiously because edges cut skin. Major sources include Mexico, the western United States, Armenia, Ethiopia, and Japan.
