igneous
Black polished stone, likely black obsidian
Obsidian; volcanic glass, predominantly silica-rich rhyolitic composition
AI-generated identification · may be incorrectVery dark black, smooth and highly vitreous/glassy luster; noncrystalline with conchoidal fracture. Hardness about 5–5.5 Mohs; cleavage absent; specific gravity approximately 2.35–2.แ4.
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Physical properties
Very dark black, smooth and highly vitreous/glassy luster; noncrystalline with conchoidal fracture. Hardness about 5–5.5 Mohs; cleavage absent; specific gravity approximately 2.35–2.แ4.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, generally in volcanic domes and flows, mostly younger than a few million years. The image appears to show a polished or tumbled piece.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically extremely sharp cutting tools; not typically used as structural stone. Common polished specimens are inexpensive.
Geological facts
Obsidian may display sheen, banding, or snowflake-like cristobalite inclusions, none clearly visible here. Similar-looking black stones include basalt, onyx, tourmaline, and manufactured glass.
Field identification & locations
Common in volcanic regions such as Iceland, Mexico, the western United States, Armenia, and Japan. Check for curved conchoidal fracture and glassy edges; a gemological or refractive-index test is needed for certainty.
