igneous
Obsidian (volcanic glass)
Obsidian, natural rhyolitic glass; predominantly SiO₂
AI-generated identification · may be incorrectDark brown to black with a glossy, glassy luster and curved, sharp conchoidal fractures; typically Mohs 5–5.5, with no cleavage or crystal structure. The pictured piece appears naturally fractured and may be slightly translucent at thin edges.
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Physical properties
Dark brown to black with a glossy, glassy luster and curved, sharp conchoidal fractures; typically Mohs 5–5.5, with no cleavage or crystal structure. The pictured piece appears naturally fractured and may be slightly translucent at thin edges.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow; most deposits are geologically young and associated with volcanic regions.
Uses & applications
Used for ornamental objects, jewelry, and historically for exceptionally sharp cutting tools; some varieties are collected as specimens.
Geological facts
Obsidian is technically volcanic glass rather than a crystalline mineral. Its fractures can produce edges sharper than many steel blades.
Field identification & locations
Identify by its glassy sheen, smooth curved fracture surfaces, and razor-sharp edges; common near rhyolitic volcanoes, including locations in the western United States, Mexico, and Iceland.
