igneous
Obsidian (likely rough black volcanic glass)
Obsidian; predominantly rhyolitic volcanic glass, commonly near SiO₂-rich composition
AI-generated identification · may be incorrectTypically black to dark gray, vitreous luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, with poor cleavage and specific gravity around 2.3–2.6. The specimen appears partly weathered or coated, with a lighter brown-gray matrix or rind.
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Physical properties
Typically black to dark gray, vitreous luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, with poor cleavage and specific gravity around 2.3–2.6. The specimen appears partly weathered or coated, with a lighter brown-gray matrix or rind.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, usually during Quaternary volcanic eruptions, though older deposits occur. Commonly associated with rhyolite, pumice, and perlite.
Uses & applications
Used for ornamental objects, cabochons, carvings, tools, and historically extremely sharp blades; attractive varieties are collected and occasionally used in jewelry.
Geological facts
Fresh obsidian breaks into exceptionally sharp edges and may show sheen, snowflake patterns, banding, or rainbow colors. It is volcanic glass rather than a true crystalline mineral.
Field identification & locations
Identify by its glassy surface, sharp conchoidal fractures, lack of crystal grains, and usually dark color; common localities include Mexico, western United States, Armenia, Turkey, and Japan. The visible alteration makes certainty from the photo limited.
