igneous
Black obsidian (polished/tumbled)
Volcanic glass; primarily silica-rich rhyolitic glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectTypically jet-black, vitreous, and translucent only at thin edges; conchoidal fracture, no cleavage, amorphous rather than crystalline, hardness about 5–5.5 Mohs, and specific gravity roughly 2.35–2.65.
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Physical properties
Typically jet-black, vitreous, and translucent only at thin edges; conchoidal fracture, no cleavage, amorphous rather than crystalline, hardness about 5–5.5 Mohs, and specific gravity roughly 2.35–2.65.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly during Quaternary volcanic eruptions. It occurs in young volcanic flows and domes, especially around rhyolitic volcanoes.
Uses & applications
Used for ornamental carvings, cabochons, beads, decorative objects, and historically extremely sharp cutting tools and arrowheads. Common polished pieces have modest value.
Geological facts
Its glassy surface can produce exceptionally sharp edges, sharper than many steel blades at microscopic scale. Snowflake obsidian contains white cristobalite spherulites, but this specimen appears uniformly black.
Field identification & locations
Identify it by its smooth glassy luster, uniform dark color, conchoidal fracture, and lack of visible grains; unlike black quartz or tourmaline, it is noncrystalline. Major sources include Mexico, the western United States, Armenia, Turkey, and Japan.
