igneous
Possible obsidian (black volcanic glass)
Obsidian; natural silica-rich volcanic glass, composition typically rhyolitic
AI-generated identification · may be incorrectDark brown to black with a glassy luster and curved, conchoidal fracture; Mohs hardness about 5–5.5. It has no crystal structure or cleavage. The photo alone cannot confirm it; slag or another dark glassy rock can look similar.
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Physical properties
Dark brown to black with a glassy luster and curved, conchoidal fracture; Mohs hardness about 5–5.5. It has no crystal structure or cleavage. The photo alone cannot confirm it; slag or another dark glassy rock can look similar.
Formation & geological history
Forms when silica-rich lava cools too quickly for crystals to grow, usually in or near volcanic flows. Its age depends on the source deposit and cannot be determined from the image.
Uses & applications
Used for ornaments, jewelry, and sharp-edged tools; historically shaped into blades and projectile points. Rough common pieces have modest collector value.
Geological facts
Obsidian is technically volcanic glass rather than a crystalline mineral, and its sharp fracture can produce exceptionally keen edges.
Field identification & locations
Look for a uniformly glassy surface and curved, shell-like fracture edges; geological context near volcanic deposits helps distinguish it from industrial slag. Found in volcanic regions including the western United States, Mexico, Iceland, and Japan.
