igneous
Obsidian (black volcanic glass)
Obsidian; predominantly rhyolitic volcanic glass, chiefly SiO₂
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; black to smoky gray, vitreous luster, conchoidal fracture, no cleavage, and noncrystalline/amorphous structure. The sharp curved edges and glassy surfaces are characteristic.
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Physical properties
Hardness about 5–5.5 Mohs; black to smoky gray, vitreous luster, conchoidal fracture, no cleavage, and noncrystalline/amorphous structure. The sharp curved edges and glassy surfaces are characteristic.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary to recent volcanic eruptions. Common in rhyolitic lava flows, domes, and volcanic margins.
Uses & applications
Used for tools, blades, arrowheads, ornaments, cabochons, and decorative objects; historically valued for exceptionally sharp cutting edges. Usually inexpensive as a common specimen.
Geological facts
Obsidian is technically volcanic glass rather than a true mineral because it lacks an ordered crystal lattice. Hydration and devitrification can gradually produce microscopic crystals over geological time.
Field identification & locations
Identify by its glassy appearance, conchoidal fracture, uniform dark color, and sharp edges; it is commonly found in volcanic regions such as Mexico, the western United States, Armenia, Iceland, and Japan. Avoid handling freshly fractured edges.
