igneous
Obsidian (black volcanic glass)
Obsidian; predominantly silica-rich volcanic glass, commonly rhyolitic (approximately 70–75% SiO₂)
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; typically jet-black, vitreous, and noncrystalline with conchoidal fracture and razor-sharp edges. The glossy surface and curved fracture scars are characteristic; iron-oxide or feldspar inclusions may produce brown or gray patches.
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Physical properties
Hardness about 5–5.5 Mohs; typically jet-black, vitreous, and noncrystalline with conchoidal fracture and razor-sharp edges. The glossy surface and curved fracture scars are characteristic; iron-oxide or feldspar inclusions may produce brown or gray patches.
Formation & geological history
Forms when silica-rich lava cools extremely rapidly, preventing crystal growth, usually in volcanic domes and lava flows. Most specimens are Quaternary to Cenozoic, though obsidian occurs wherever relatively young rhyolitic volcanism took place.
Uses & applications
Used historically for arrowheads, knives, and other cutting tools; today it is collected, carved, and occasionally used in jewelry and decorative objects. It has little construction value compared with crystalline volcanic rocks.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Hydration-rim dating can sometimes estimate the age of archaeological artifacts.
Field identification & locations
Identify it by its glassy black luster, lack of visible crystals, and conchoidal fracture; test cautiously because edges are very sharp. Common localities include Iceland, Mexico, the western United States, Armenia, and Japan.
