igneous
Black tumbled stone, possibly obsidian
Volcanic glass; primarily SiO₂, if obsidian
AI-generated identification · may be incorrectBlack, opaque-to-slightly translucent, vitreous luster with irregular conchoidal-looking surfaces; obsidian has no crystal structure and Mohs hardness about 5–5.5. The photograph is insufficient to distinguish obsidian from black tourmaline, basalt, or another polished dark mineral.
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Physical properties
Black, opaque-to-slightly translucent, vitreous luster with irregular conchoidal-looking surfaces; obsidian has no crystal structure and Mohs hardness about 5–5.5. The photograph is insufficient to distinguish obsidian from black tourmaline, basalt, or another polished dark mineral.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in volcanic settings younger than a few million years. Similar-looking basaltic stones form from crystalline mafic lava.
Uses & applications
Used for ornamental carvings, beads, cabochons, tools, and collectors’ specimens; obsidian historically provided exceptionally sharp cutting edges. Basalt is more commonly used as aggregate and construction stone.
Geological facts
Obsidian can show sheen, snowflake patterns, or rainbow colors caused by microscopic inclusions or flow structure. It breaks with sharp conchoidal edges rather than displaying mineral cleavage.
Field identification & locations
Look for glassy luster, uniform black color, sharp curved fracture, and possible translucence at thin edges; a streak, hardness, and close inspection are needed for confirmation. Common sources include Mexico, the western United States, Iceland, and Armenia.
