igneous
Obsidian (likely volcanic glass)
Obsidian; predominantly rhyolitic glass, chiefly SiO₂
AI-generated identification · may be incorrectBlack to dark gray, translucent greenish-brown edges, vitreous luster, amorphous (noncrystalline), no cleavage, conchoidal fracture, and Mohs hardness about 5–5.5. The sharp curved edges and glassy surface are characteristic.
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Physical properties
Black to dark gray, translucent greenish-brown edges, vitreous luster, amorphous (noncrystalline), no cleavage, conchoidal fracture, and Mohs hardness about 5–5.5. The sharp curved edges and glassy surface are characteristic.
Formation & geological history
Rapid cooling of silica-rich lava prevented crystal growth, commonly during volcanic eruptions. Most specimens formed in the Quaternary or older Cenozoic, though obsidian occurs through much of geologic time.
Uses & applications
Used historically for blades, arrowheads, and mirrors; today it is collected, carved, and occasionally used decoratively or in jewelry. It is not generally used as structural stone.
Geological facts
Obsidian can fracture to edges sharper than surgical steel, although they are brittle. The greenish translucent area may reflect weathering, internal flow banding, or a natural color variation.
Field identification & locations
Identify by its glassy luster, lack of visible crystals, conchoidal fracture, and unusually sharp edges; test cautiously because flakes can cut skin. Common sources include Mexico, the western United States, Armenia, Turkey, and Japan.
