igneous
Obsidian (black volcanic glass)
Obsidian; predominantly rhyolitic volcanic glass, mainly SiO₂ with minor Al, K, Na, Fe, and Mg
AI-generated identification · may be incorrectUsually black, vitreous/glassy, and noncrystalline, with conchoidal fracture and sharp edges; Mohs hardness about 5–5.5, no cleavage, specific gravity about 2.35–2.65. The specimen’s glossy black surface and curved fracture strongly fit obsidian, though image-only identification is not definitive.
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Physical properties
Usually black, vitreous/glassy, and noncrystalline, with conchoidal fracture and sharp edges; Mohs hardness about 5–5.5, no cleavage, specific gravity about 2.35–2.65. The specimen’s glossy black surface and curved fracture strongly fit obsidian, though image-only identification is not definitive.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, typically during Quaternary to older volcanic eruptions. Common around rhyolitic volcanic domes, flows, and ash deposits.
Uses & applications
Used historically for blades, arrowheads, and tools because it fractures to exceptionally sharp edges; today it is collected, carved, used in jewelry, and occasionally as decorative stone.
Geological facts
Fresh fracture surfaces can be sharper than surgical steel; hydration-rind dating can estimate the age of archaeological obsidian artifacts.
Field identification & locations
Identify by its glassy luster, lightweight feel, lack of visible crystals, and conchoidal fracture; common localities include Mexico, the western United States, Iceland, Armenia, and Japan. Check for bubbles, flow banding, or snowflake-like cristobalite inclusions.
