igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass; primarily SiO₂)
AI-generated identification · may be incorrectDark brown-black, vitreous luster, smooth polished surface, and no visible crystals or cleavage; hardness about 5–5.5 Mohs, with conchoidal fracture when unpolished. The image is insufficient to distinguish obsidian confidently from black jasper, basalt, or tourmaline.
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Physical properties
Dark brown-black, vitreous luster, smooth polished surface, and no visible crystals or cleavage; hardness about 5–5.5 Mohs, with conchoidal fracture when unpolished. The image is insufficient to distinguish obsidian confidently from black jasper, basalt, or tourmaline.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth; most examples are geologically young, commonly Quaternary to Cenozoic. It occurs around volcanic domes and lava flows.
Uses & applications
Used for ornamental carvings, beads, cabochons, knife blades, and collector specimens; historically, sharp obsidian flakes were used as cutting tools.
Geological facts
Its glassy texture and conchoidal fracture are diagnostic; tiny gas bubbles or flow bands may be visible under magnification. A polished surface can obscure important identifying features.
Field identification & locations
Common localities include Mexico, the western United States, Armenia, Iceland, and Italy. Check for glass-like translucency at thin edges and conchoidal chips; a laboratory test is needed for certainty.
