igneous
Tumbled black stone, likely black obsidian
Obsidian (volcanic glass; chiefly silica-rich rhyolitic composition)
AI-generated identification · may be incorrectTypically black, glassy/vitreous, and noncrystalline, with conchoidal fracture; Mohs hardness about 5–5.5 and specific gravity about 2.35–2.65. The specimen appears polished or tumbled, with tiny reflective inclusions or bubbles.
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Physical properties
Typically black, glassy/vitreous, and noncrystalline, with conchoidal fracture; Mohs hardness about 5–5.5 and specific gravity about 2.35–2.65. The specimen appears polished or tumbled, with tiny reflective inclusions or bubbles.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly in volcanic flows and domes; ages range from recent to millions of years. Exact identification is uncertain from the image alone.
Uses & applications
Used for beads, cabochons, carvings, decorative objects, and historically sharp blades and arrowheads; not generally used as a structural stone.
Geological facts
Obsidian is technically volcanic glass rather than a crystalline mineral. Similar black tumbled material may be black jasper, basalt, or man-made glass.
Field identification & locations
Look for a uniformly glassy surface and sharp conchoidal edges; obsidian commonly occurs in volcanic regions such as Mexico, the western United States, Armenia, Turkey, and Japan. A gemological or laboratory test is needed for confirmation.
