igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass), chiefly SiO₂
AI-generated identification · may be incorrectVery dark black, vitreous luster, apparently rounded and polished; hardness about 5–5.5 Mohs, conchoidal fracture, no visible cleavage, and specific gravity about 2.35–2.เจ5. Small reflective patches may indicate devitrification or surface coating; exact identification is uncertain from the image alone.
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Physical properties
Very dark black, vitreous luster, apparently rounded and polished; hardness about 5–5.5 Mohs, conchoidal fracture, no visible cleavage, and specific gravity about 2.35–2.เจ5. Small reflective patches may indicate devitrification or surface coating; exact identification is uncertain from the image alone.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, typically in volcanic settings during the Quaternary and older Cenozoic. This specimen appears human-shaped or tumbled rather than naturally rounded.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically extremely sharp cutting tools; not generally used as structural stone. Common polished pieces usually have modest collector value.
Geological facts
Obsidian is technically a natural volcanic glass rather than a crystalline mineral. It can weather or devitrify into fine-grained rhyolitic material over time.
Field identification & locations
Check for glassy conchoidal fracture, sharp edges when broken, uniform density, and absence of bubbles or molded seams; a streak test is usually white or colorless. Common sources include Mexico, the western United States, Armenia, Turkey, and Japan.
