igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass), chiefly SiO₂; possibly black tourmaline or onyx
AI-generated identification · may be incorrectTypically glassy/vitreous, opaque black with conchoidal fracture and no visible crystals; hardness about 5–5.5 Mohs, specific gravity ~2.35–2.60. The uniformly rounded, highly polished form prevents definitive identification from the image alone.
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Physical properties
Typically glassy/vitreous, opaque black with conchoidal fracture and no visible crystals; hardness about 5–5.5 Mohs, specific gravity ~2.35–2.60. The uniformly rounded, highly polished form prevents definitive identification from the image alone.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystallization, generally in volcanic settings younger than a few million years. Black tourmaline and onyx are alternative possibilities requiring testing.
Uses & applications
Used for ornamental objects, cabochons, beads, and historically sharp tools; common polished specimens have modest value.
Geological facts
Obsidian can preserve flow bands, bubbles, or a sheen, while onyx is cryptocrystalline quartz and tourmaline is crystalline borosilicate. A glassy luster and conchoidal fracture favor obsidian.
Field identification & locations
Common localities include Mexico, the western United States, Armenia, Iceland, and Japan. Check streak, hardness, magnification, and fracture; obsidian will not scratch easily with a steel knife but can be scratched by quartz.
