igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass; predominantly silica-rich SiO₂)
AI-generated identification · may be incorrectTypically black, vitreous and noncrystalline, with conchoidal fracture and Mohs hardness about 5–5.5; opaque appearance may reflect iron- or magnetite-rich glass. The smooth, glossy surface indicates polishing or tumbling.
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Physical properties
Typically black, vitreous and noncrystalline, with conchoidal fracture and Mohs hardness about 5–5.5; opaque appearance may reflect iron- or magnetite-rich glass. The smooth, glossy surface indicates polishing or tumbling.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in Quaternary to older volcanic regions. The image alone cannot distinguish obsidian from polished basalt, jet, or industrial glass.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically extremely sharp blades; common polished pieces have modest decorative value.
Geological facts
Obsidian is technically volcanic glass rather than a crystalline mineral. Black varieties may show subtle sheen, flow banding, or “snowflake” cristobalite inclusions when viewed under strong light.
Field identification & locations
Check for curved conchoidal edges, glassy luster, and absence of visible grains; obsidian commonly occurs near rhyolitic lava flows in Mexico, the western United States, Iceland, Italy, and Japan. A hardness or streak test can help, but testing may damage a polished specimen.
