igneous
Obsidian (volcanic glass)
Obsidian; naturally occurring silica-rich volcanic glass, typically rhyolitic
AI-generated identification · may be incorrectDark brown to black, glossy vitreous luster, and conchoidal fracture with sharp edges; hardness about 5–5.5 Mohs. Amorphous rather than crystalline; the small colorful glint may be reflected light or an iridescent variety.
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Physical properties
Dark brown to black, glossy vitreous luster, and conchoidal fracture with sharp edges; hardness about 5–5.5 Mohs. Amorphous rather than crystalline; the small colorful glint may be reflected light or an iridescent variety.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystals from growing. It is generally geologically young because weathering and hydration gradually alter it.
Uses & applications
Used for ornaments, jewelry, and collectible specimens; historically knapped into exceptionally sharp blades and tools.
Geological facts
Obsidian can fracture into edges sharper than many steel blades, though those edges are brittle.
Field identification & locations
Look for glassy luster, sharp curved fracture surfaces, and absence of visible mineral grains. Common sources include volcanic regions such as the western United States, Mexico, Iceland, and Japan; the image alone cannot establish the exact variety.
