igneous
Tumbled black stone, likely obsidian
Obsidian — volcanic glass, chiefly silica-rich rhyolitic composition
AI-generated identification · may be incorrectBlack to dark gray, smooth rounded polish, vitreous luster, noncrystalline glass with conchoidal fracture; Mohs hardness about 5–5.5 and specific gravity roughly 2.3–2.6. The image is not sufficient to exclude black glass, onyx, or other dark stones.
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Physical properties
Black to dark gray, smooth rounded polish, vitreous luster, noncrystalline glass with conchoidal fracture; Mohs hardness about 5–5.5 and specific gravity roughly 2.3–2.6. The image is not sufficient to exclude black glass, onyx, or other dark stones.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, usually in volcanic domes and flows; commonly Quaternary to older Cenozoic, though obsidian occurs throughout geological time. Common sources include Mexico, the western United States, Armenia, Italy, and Japan.
Uses & applications
Used for ornaments, cabochons, beads, carvings, cutting tools, and historically exceptionally sharp blades; not generally used as structural aggregate. Common tumbled specimens typically have modest collector value.
Geological facts
Fresh fracture surfaces are sharply conchoidal and often mirrorlike; snowflake, mahogany, rainbow, and sheen varieties result from inclusions or optical effects. Obsidian is technically a mineraloid rather than a true mineral.
Field identification & locations
Identify by its glassy luster, lack of visible grains, light weight relative to many dark rocks, and sharp conchoidal chips; a streak test is usually absent or faint. Confirmatory testing requires examining an unpolished edge or laboratory analysis.
