igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass), predominantly silica-rich rhyolitic composition
AI-generated identification · may be incorrectTypically black, with a smooth vitreous luster and conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and noncrystalline glassy structure. The rounded, polished form obscures diagnostic fracture features; alternatives include black jasper or dyed agate.
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Physical properties
Typically black, with a smooth vitreous luster and conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and noncrystalline glassy structure. The rounded, polished form obscures diagnostic fracture features; alternatives include black jasper or dyed agate.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, generally in Quaternary to older volcanic environments. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for carvings, ornamental objects, cabochons, jewelry, and historically extremely sharp cutting tools. It is mainly a decorative and collectible material rather than a construction stone.
Geological facts
Obsidian can produce exceptionally sharp edges because it fractures conchoidally. Some varieties display golden sheen, rainbow colors, or snowflake-like cristobalite inclusions.
Field identification & locations
Identify by its glassy luster, uniform dark color, light weight relative to many opaque stones, and conchoidal fracture if an unpolished edge is available; a scratch or streak test can help distinguish jasper. Common polished specimens usually lack locality clues.
