volcanic glass (igneous)
Black tumbled stone, possibly obsidian
Obsidian, naturally occurring rhyolitic glass; identification is uncertain from the image alone
AI-generated identification · may be incorrectSmall, dark, polished pebble with a glassy luster; obsidian has no crystals or cleavage, conchoidal fracture, Mohs hardness about 5–5.5, and specific gravity around 2.3–2.6.
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Physical properties
Small, dark, polished pebble with a glassy luster; obsidian has no crystals or cleavage, conchoidal fracture, Mohs hardness about 5–5.5, and specific gravity around 2.3–2.6.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth; deposits range from young volcanic flows to ancient sources. The pictured piece may have been tumbled or polished.
Uses & applications
Used for ornaments, jewelry, and historically sharp blades; polished pieces are popular with collectors.
Geological facts
Obsidian can break into exceptionally sharp edges. A photo cannot reliably distinguish it from other dark polished minerals or glass.
Field identification & locations
Look for glassy sheen and curved, shell-like fracture surfaces; common sources include volcanic regions such as Mexico, the western United States, and Iceland.
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