igneous
Tumbled black stone, likely obsidian
Obsidian (volcanic glass; predominantly SiO₂)
AI-generated identification · may be incorrectBlack to dark brown, smooth and glassy with vitreous luster; conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5. The rounded shape indicates tumbling or water wear; the image alone cannot exclude black jasper, basalt, or slag.
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Physical properties
Black to dark brown, smooth and glassy with vitreous luster; conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5. The rounded shape indicates tumbling or water wear; the image alone cannot exclude black jasper, basalt, or slag.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in volcanic regions younger than a few million years. It is commonly associated with rhyolitic lava flows and domes.
Uses & applications
Used historically for exceptionally sharp blades and arrowheads, and today for carvings, jewelry, cabochons, and decorative objects. Common tumbled pieces have modest value.
Geological facts
Obsidian is technically a natural volcanic glass rather than a crystalline mineral. Brown or golden internal flashes may reflect flow banding or inclusions.
Field identification & locations
Check for sharp conchoidal edges, glassy luster, and lack of crystal grains; obsidian is usually lighter than magnetite and does not strongly attract a magnet. Common sources include Mexico, the western United States, Armenia, and Turkey.
