igneous
Polished/tumbled stone, likely mahogany obsidian
Obsidian, natural volcanic glass (composition typically rhyolitic)
AI-generated identification · may be incorrectSmooth, glossy dark brown-black surface with reddish-brown patches; obsidian has no crystal structure or cleavage and is about 5–5.5 Mohs. The photo alone cannot confirm the identification; the stone may be another polished dark rock.
Identified More igneous →
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Physical properties
Smooth, glossy dark brown-black surface with reddish-brown patches; obsidian has no crystal structure or cleavage and is about 5–5.5 Mohs. The photo alone cannot confirm the identification; the stone may be another polished dark rock.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystals from growing; it is generally geologically young, commonly associated with Quaternary volcanism.
Uses & applications
Used for decorative objects, jewelry, and collectible carvings; historically shaped into sharp tools.
Geological facts
Obsidian breaks with a curved, shell-like fracture and can produce exceptionally sharp edges.
Field identification & locations
Look for glassy luster and conchoidal fracture on an unpolished edge; mahogany obsidian is known from volcanic regions including Mexico and the western United States. This rounded specimen’s identity is uncertain from the image.
