igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass), chiefly silica-rich rhyolitic composition
AI-generated identification · may be incorrectBlack, opaque, and highly vitreous with a glassy luster; hardness about 5–5.5 Mohs, conchoidal fracture, no cleavage, and typically amorphous rather than crystalline. The uniformly smooth surface indicates polishing or tumbling.
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Physical properties
Black, opaque, and highly vitreous with a glassy luster; hardness about 5–5.5 Mohs, conchoidal fracture, no cleavage, and typically amorphous rather than crystalline. The uniformly smooth surface indicates polishing or tumbling.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary to recent volcanic eruptions. Common sources include Mexico, the western United States, Iceland, Armenia, and parts of Japan.
Uses & applications
Used for ornamental stones, cabochons, beads, carvings, and historically extremely sharp tools and projectile points; rarely used in construction. Value is usually modest unless it has unusual sheen, inclusions, or provenance.
Geological facts
Obsidian can produce exceptionally sharp edges because it fractures conchoidally. The image could also represent polished black basalt, onyx, or man-made glass; a close visual identification is therefore provisional.
Field identification & locations
Obsidian is identified by its glassy appearance, lack of visible grains, sharp conchoidal fracture, and light-to-medium weight; streak and hardness testing can help distinguish it from glass or basalt. Common rough material occurs near volcanic fields.
