igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass; chiefly silica-rich rhyolitic composition)
AI-generated identification · may be incorrectTypically black, vitreous/glassy luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; the rounded specimen appears polished and may have reflective inclusions or surface pits.
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Physical properties
Typically black, vitreous/glassy luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; the rounded specimen appears polished and may have reflective inclusions or surface pits.
Formation & geological history
Forms when silica-rich volcanic lava cools so rapidly that crystals cannot grow, generally in Quaternary to younger volcanic flows and domes. The image alone cannot confirm obsidian over black basalt, onyx, or slag.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically very sharp cutting tools; polished pieces have modest decorative value.
Geological facts
Obsidian can show snowflake, sheen, rainbow, or mahogany patterns caused by crystals or microscopic inclusions. Its glassy fracture produces exceptionally sharp edges.
Field identification & locations
Look for a glass-like surface, conchoidal fracture, uniform dark color, and lack of granular crystals; common sources include Mexico, the western United States, Armenia, and Japan. A streak test, magnification, and specific-gravity check would improve identification.
