igneous
Polished black stone, likely obsidian
Obsidian (volcanic glass; mostly silica-rich rhyolitic composition)
AI-generated identification · may be incorrectVery dark gray to black, vitreous/glassy luster, amorphous rather than crystalline, conchoidal fracture, and Mohs hardness about 5–5.5. The tan line may be a natural fracture, mineral vein, or surface inclusion; the image alone cannot confirm identity.
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Physical properties
Very dark gray to black, vitreous/glassy luster, amorphous rather than crystalline, conchoidal fracture, and Mohs hardness about 5–5.5. The tan line may be a natural fracture, mineral vein, or surface inclusion; the image alone cannot confirm identity.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals do not develop, commonly during Quaternary to older volcanic eruptions. It occurs in volcanic regions worldwide, including Mexico, the western United States, Iceland, Italy, and Armenia.
Uses & applications
Used for ornamental carvings, cabochons, beads, decorative objects, and historically extremely sharp blades and arrowheads; less commonly used as a construction stone.
Geological facts
Obsidian can produce exceptionally sharp edges and may display sheen, snowflake patterns, or banding from microscopic inclusions. Similar polished black materials include black jasper, basalt, onyx, and man-made glass.
Field identification & locations
Check for natural conchoidal chips, uniformly glassy luster, and lack of bubbles; a knife-edge scratch test is not recommended on polished pieces. A gemological or specific-gravity test is needed for certainty.
