igneous
Black obsidian (likely)
Volcanic glass; primarily silica-rich rhyolitic composition (SiO₂-rich, noncrystalline)
AI-generated identification · may be incorrectTypically black, vitreous, and highly reflective with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, poor cleavage, and specific gravity around 2.3–2.6. The photographed piece appears rough and glassy, though lighting limits certainty.
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Physical properties
Typically black, vitreous, and highly reflective with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, poor cleavage, and specific gravity around 2.3–2.6. The photographed piece appears rough and glassy, though lighting limits certainty.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during Quaternary volcanic eruptions. It is found near young rhyolitic lava flows and volcanic domes.
Uses & applications
Used for ornamental objects, jewelry, cabochons, tools, and historically razor-sharp blades; construction use is limited.
Geological facts
Fresh fracture edges can be exceptionally sharp. Snowflake obsidian contains white cristobalite spherulites, while sheen varieties show reflective bands or patches.
Field identification & locations
Identify by its glassy luster, curved shell-like fracture, uniform dark color, and lack of grain; volcanic slag and industrial glass can look similar. Common localities include Mexico, Arizona, Oregon, California, Armenia, and Iceland.
