igneous
Black obsidian
Volcanic glass, predominantly rhyolitic silica glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectTypically black, vitreous, and noncrystalline, with conchoidal fracture and sharp edges; Mohs hardness 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The specimen appears rough or partly fractured, possibly with subtle flow banding.
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Physical properties
Typically black, vitreous, and noncrystalline, with conchoidal fracture and sharp edges; Mohs hardness 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The specimen appears rough or partly fractured, possibly with subtle flow banding.
Formation & geological history
Forms when silica-rich lava cools so quickly that crystals cannot develop, commonly around volcanic domes and lava flows. Most examples are Quaternary to Cenozoic, though obsidian occurs in many volcanic ages.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically extremely sharp cutting tools and arrowheads; generally unsuitable for structural construction.
Geological facts
Its glassy texture distinguishes it from crystalline basalt; translucent brown, gray, or rainbow varieties can occur. Fresh fractures may be sharper than surgical steel edges, though they are brittle.
Field identification & locations
Identify by glassy luster, curved shell-like fracture, uniform dark color, and lack of visible crystals; common localities include Mexico, the western United States, Armenia, Turkey, and Iceland. Value is usually $5–$30 for a small common specimen, or $30–$150+ for attractive polished or unusual varieties.
