igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass), primarily silica-rich rhyolitic composition
AI-generated identification · may be incorrectBlack, glossy to vitreous luster with conchoidal fracture; amorphous rather than crystalline, Mohs hardness about 5–5.5, no cleavage, specific gravity about 2.35–2.65. The visible gray speckling may indicate snowflake obsidian or another dark volcanic rock such as basalt.
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Physical properties
Black, glossy to vitreous luster with conchoidal fracture; amorphous rather than crystalline, Mohs hardness about 5–5.5, no cleavage, specific gravity about 2.35–2.65. The visible gray speckling may indicate snowflake obsidian or another dark volcanic rock such as basalt.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically in young volcanic regions; most deposits are Cenozoic or younger, though older obsidian also occurs. Exact identification is limited by the photograph.
Uses & applications
Used for ornamental objects, cabochons, beads, arrowheads, knives, and polished palm stones; historically fashioned into exceptionally sharp cutting tools.
Geological facts
Obsidian breaks into extremely sharp edges and can appear nearly mirror-black when polished. Snowflake patterns result from devitrified cristobalite or feldspar inclusions.
Field identification & locations
Check for curved, shell-like fracture, glassy shine, uniform density, and lack of visible grains; a steel file may scratch it slightly. Common locations include Mexico, the western United States, Armenia, Turkey, and Japan.
