igneous
Obsidian (volcanic glass)
Obsidian; predominantly silica-rich volcanic glass, commonly rhyolitic in composition
AI-generated identification · may be incorrectUsually black with a glassy luster and conchoidal, razor-sharp fracture; hardness about 5–5.5 Mohs. Amorphous (no crystal structure); the specimen appears naturally fractured, though its glossy surfaces may be shaped.
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Physical properties
Usually black with a glassy luster and conchoidal, razor-sharp fracture; hardness about 5–5.5 Mohs. Amorphous (no crystal structure); the specimen appears naturally fractured, though its glossy surfaces may be shaped.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically during volcanic eruptions. Obsidian occurs in deposits of many ages, often associated with relatively young volcanic regions.
Uses & applications
Historically used for sharp tools and weapons; today used in jewelry, ornaments, and collectible specimens.
Geological facts
Obsidian is natural volcanic glass, not a mineral, because it lacks an ordered crystal structure.
Field identification & locations
Identify by its glassy sheen, black color, and curved conchoidal fracture; common in volcanic areas such as the western United States, Mexico, Iceland, and the Andes. The image alone cannot establish the exact locality or whether the piece was worked.
