igneous
Black tumbled stone, likely obsidian
Volcanic glass (obsidian), primarily amorphous SiO₂
AI-generated identification · may be incorrectTypically black with vitreous luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; density is approximately 2.3–2.6. The smooth rounded surface indicates tumbling or polishing.
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Physical properties
Typically black with vitreous luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; density is approximately 2.3–2.6. The smooth rounded surface indicates tumbling or polishing.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, usually in volcanic flows or domes from the Quaternary and older Cenozoic eras. The photograph alone cannot exclude black glass, basalt, or another polished stone.
Uses & applications
Commonly sold as a low-cost lapidary, meditation, and ornamental stone; historically used for exceptionally sharp tools and blades. It is not generally used as structural stone.
Geological facts
Fresh obsidian breaks with very sharp conchoidal edges and may show gray, brown, rainbow, or golden sheen caused by inclusions or microscopic structures.
Field identification & locations
Check for a glassy luster, conchoidal fracture, lack of grain, and a streakless surface; a scratch test and density test help distinguish it from basalt or manufactured glass. Common sources include Mexico, the western United States, Iceland, Armenia, and Japan.
