igneous
Tumbled obsidian
Volcanic glass; chiefly SiO₂, noncrystalline
AI-generated identification · may be incorrectBlack to dark green, highly vitreous/glassy luster with conchoidal fracture; hardness about 5–5.5 Mohs, no cleavage, specific gravity about 2.35–2.60. The smooth rounded surface indicates tumbling or polishing; blue-green color may reflect lighting or surface treatment.
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Physical properties
Black to dark green, highly vitreous/glassy luster with conchoidal fracture; hardness about 5–5.5 Mohs, no cleavage, specific gravity about 2.35–2.60. The smooth rounded surface indicates tumbling or polishing; blue-green color may reflect lighting or surface treatment.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly at volcanic domes and lava flows during the Quaternary and older volcanic episodes. It is not technically a crystalline mineral because it lacks an ordered crystal structure.
Uses & applications
Used for ornaments, cabochons, carvings, arrowheads, cutting tools, and jewelry; exceptionally sharp experimental blades can be made from fresh obsidian.
Geological facts
Its conchoidal fracture can produce edges sharper than surgical steel, though they are fragile. Snowflake, rainbow, mahogany, and sheen varieties are common decorative forms.
Field identification & locations
Identify by glassy luster, uniform dark color, curved fracture patterns, and lack of visible grains or cleavage; common sources include Mexico, the western United States, Armenia, Turkey, and Japan. A polished black glass-like specimen could also be slag or manufactured glass, so locality or a fracture test helps.
