igneous
Obsidian (black volcanic glass)
Obsidian; silica-rich volcanic glass, typically rhyolitic to dacitic composition
AI-generated identification · may be incorrectUsually black, glossy to waxy, with conchoidal fracture and no visible crystals; hardness about 5–5.5 Mohs, glassy luster, brittle, and specific gravity about 2.3–2.6. The specimen’s irregular shiny surfaces and sharp-looking edges are consistent with obsidian, though lighting or a surface coating may affect appearance…
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Physical properties
Usually black, glossy to waxy, with conchoidal fracture and no visible crystals; hardness about 5–5.5 Mohs, glassy luster, brittle, and specific gravity about 2.3–2.6. The specimen’s irregular shiny surfaces and sharp-looking edges are consistent with obsidian, though lighting or a surface coating may affect appearance.
Formation & geological history
Forms when silica-rich lava cools extremely rapidly, preventing crystal growth; most examples are geologically young, commonly Quaternary to Cenozoic. It occurs in volcanic domes, lava flows, and rhyolitic calderas.
Uses & applications
Used for ornamental objects, cabochons, carving, tools, and historically exceptionally sharp blades; unsuitable as a major construction aggregate because it fractures readily.
Geological facts
Obsidian is technically a natural glass rather than a crystalline mineral. Hydration-rim dating can sometimes estimate its surface exposure age.
Field identification & locations
Identify by its glassy appearance, conchoidal fracture, sharp edges, and lack of mineral grains; common localities include Armenia, Mexico, the western United States, Iceland, Italy, and Japan. Value is typically $5–$30 for a common rough piece, or $30–$150+ for attractive quality or unusual patterns.
