igneous
Black tumbled stone, possibly obsidian
Obsidian (volcanic glass; predominantly SiO₂), identification uncertain from image
AI-generated identification · may be incorrectSmall, uniformly black, smooth, polished fragment with likely vitreous luster; obsidian has no crystal structure, conchoidal fracture, Mohs hardness about 5–5.5, and specific gravity about 2.35. The dark appearance could also represent polished basalt, tourmaline, or another stone.
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Physical properties
Small, uniformly black, smooth, polished fragment with likely vitreous luster; obsidian has no crystal structure, conchoidal fracture, Mohs hardness about 5–5.5, and specific gravity about 2.35. The dark appearance could also represent polished basalt, tourmaline, or another stone.
Formation & geological history
If obsidian, it formed when silica-rich lava cooled rapidly, preventing crystal growth, commonly during Cenozoic volcanic activity. Obsidian occurs in young volcanic flows and domes, especially in the western United States, Mexico, Japan, and Iceland.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically exceptionally sharp tools and spear points. Common polished pieces have modest collecting value.
Geological facts
Obsidian is technically natural volcanic glass rather than a crystalline mineral. Its composition and appearance can resemble man-made glass, but natural specimens may show flow banding or conchoidal fracture.
Field identification & locations
Check for curved, shell-like fracture surfaces, glassy luster, and lack of visible grains; a steel blade generally does not scratch it easily. The photograph alone cannot distinguish it confidently from manufactured glass or other black stones.
