igneous
Tumbled black stone, likely black obsidian
Obsidian; volcanic glass, typically rhyolitic composition (SiO₂-rich)
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; opaque black, vitreous to waxy luster, conchoidal fracture, no cleavage, and glassy (non-crystalline) structure. The smooth surface indicates tumbling or polishing.
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Physical properties
Hardness about 5–5.5 Mohs; opaque black, vitreous to waxy luster, conchoidal fracture, no cleavage, and glassy (non-crystalline) structure. The smooth surface indicates tumbling or polishing.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, usually in young volcanic flows from the Cenozoic or younger. Common sources include Mexico, the western United States, Armenia, and Iceland.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically extremely sharp blades and tools; it has limited construction use.
Geological facts
Obsidian can produce edges sharper than surgical steel but is brittle. The uniformly glossy black appearance may also represent polished basalt, black jasper, or man-made glass, which cannot be distinguished confidently from this photo alone.
Field identification & locations
Obsidian commonly shows conchoidal fracture, sharp edges, and occasional flow bands or tiny bubbles; basalt is usually grainier and less glassy. A streak, magnification, and specific-gravity test can help distinguish alternatives.
