igneous
Polished black stone, likely black obsidian
Obsidian — volcanic glass, chiefly SiO₂
AI-generated identification · may be incorrectTypically black, smooth, and glassy with vitreous luster; hardness about 5–5.5 Mohs, no crystal structure or cleavage, and conchoidal fracture. The rounded, polished surface prevents confident identification; black agate, onyx, or tourmaline are alternatives.
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Physical properties
Typically black, smooth, and glassy with vitreous luster; hardness about 5–5.5 Mohs, no crystal structure or cleavage, and conchoidal fracture. The rounded, polished surface prevents confident identification; black agate, onyx, or tourmaline are alternatives.
Formation & geological history
Rapid cooling of silica-rich lava produced volcanic glass, generally in the Quaternary or older volcanic settings. Commonly associated with rhyolitic lava flows and domes.
Uses & applications
Used for ornamental spheres, carvings, jewelry, and historically razor-sharp tools and arrowheads. This appears to be a small polished decorative specimen.
Geological facts
Obsidian can fracture into extremely sharp edges and may show subtle sheen, banding, or snowflake cristobalite inclusions. A glassy appearance alone is not diagnostic after polishing.
Field identification & locations
Common sources include Iceland, Mexico, the western United States, Armenia, and Japan. Confirm with an unpolished edge, streak testing, and examination for conchoidal fracture; avoid destructive tests on polished pieces.
