igneous
Polished black obsidian
Volcanic glass; primarily rhyolitic SiO₂, with minor iron, magnesium, and other trace elements
AI-generated identification · may be incorrectTypically black, vitreous, and noncrystalline, with conchoidal fracture and no cleavage; Mohs hardness about 5–5.5 and specific gravity about 2.35–2.65. The glossy surface indicates polishing; the piece may also be shaped or tumbled.
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Physical properties
Typically black, vitreous, and noncrystalline, with conchoidal fracture and no cleavage; Mohs hardness about 5–5.5 and specific gravity about 2.35–2.65. The glossy surface indicates polishing; the piece may also be shaped or tumbled.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly at volcanic domes and lava flows. Most specimens are Quaternary to older Cenozoic, though obsidian occurs throughout geologic time.
Uses & applications
Used for ornamental carvings, cabochons, beads, knives, arrowheads, and decorative objects; exceptionally sharp experimental blades can be made from its fracture edges.
Geological facts
Obsidian is technically natural glass rather than a crystalline mineral. Some varieties show sheen, snowflake patterns, or rainbow colors caused by inclusions or microscopic structures.
Field identification & locations
Identify by its uniformly glassy black appearance, curved conchoidal fractures, lack of visible grains, and sharp edges; common localities include Mexico, the western United States, Armenia, Iceland, and Japan. A scratch test and examination of an unpolished edge help distinguish it from black glass or plastic.
