igneous
Polished obsidian
Volcanic glass; predominantly rhyolitic SiO₂
AI-generated identification · may be incorrectTypically black to dark gray with a vitreous luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5 and specific gravity about 2.35–2.65. The specimen appears polished and may show gray flow banding or sheen.
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Physical properties
Typically black to dark gray with a vitreous luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5 and specific gravity about 2.35–2.65. The specimen appears polished and may show gray flow banding or sheen.
Formation & geological history
Forms when silica-rich lava cools extremely rapidly, preventing crystallization; most examples are Quaternary to recent in age and occur around young volcanic fields.
Uses & applications
Used for ornamental objects, cabochons, beads, knives, arrowheads, and lapidary carvings; historically valued for exceptionally sharp cutting edges.
Geological facts
Obsidian is technically volcanic glass rather than a fully crystalline rock and can weather into perlite or devitrified material over time.
Field identification & locations
Identify by its glassy luster, sharp conchoidal edges, lack of crystals, and uniform dark color; common sources include Mexico, the western United States, Armenia, Iceland, and Japan. The pictured piece is consistent with polished dark obsidian, though a photo cannot exclude man-made glass.
