igneous
Tumbled black stone, likely black obsidian
Obsidian — volcanic glass, primarily SiO₂
AI-generated identification · may be incorrectUsually black with a vitreous, glassy luster; no visible crystals or cleavage; hardness about 5–5.5 Mohs and conchoidal fracture. The image is insufficient to exclude polished basalt, black agate, or industrial glass.
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Physical properties
Usually black with a vitreous, glassy luster; no visible crystals or cleavage; hardness about 5–5.5 Mohs and conchoidal fracture. The image is insufficient to exclude polished basalt, black agate, or industrial glass.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in young volcanic regions; obsidian deposits range from thousands to millions of years old.
Uses & applications
Used for ornamental objects, cabochons, beads, tools, and historically razor-sharp blades; polished pieces are common collectibles.
Geological facts
Obsidian can produce exceptionally sharp edges because it fractures conchoidally, but it is technically mineraloid rather than a true crystalline mineral.
Field identification & locations
Look for uniformly glassy luster, curved conchoidal fracture, sharp edges, and possible flow bands or tiny bubbles; common localities include Mexico, the western United States, Armenia, Iceland, and Japan.
