igneous
Polished dark stone, likely obsidian
Volcanic glass; primarily SiO₂
AI-generated identification · may be incorrectBlack to smoky-brown, smooth and highly vitreous, with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, specific gravity 2.3–2.6. The rounded, reflective surface indicates polishing or tumbling.
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Physical properties
Black to smoky-brown, smooth and highly vitreous, with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, specific gravity 2.3–2.6. The rounded, reflective surface indicates polishing or tumbling.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, typically in volcanic flows only tens of thousands to millions of years old. The exact specimen locality cannot be determined from the image.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads, and today for jewelry, cabochons, carvings, ornaments, and collectors’ specimens.
Geological facts
Obsidian is technically a natural volcanic glass rather than a crystalline mineral. Brown or gray translucent zones can occur, and weathering may produce a dull or frosted surface.
Field identification & locations
Identify by its glassy luster, uniform dark color, lack of grain, and shell-like fracture; a glass or steel scratch test can help, though polishing obscures diagnostic edges. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
