igneous
Obsidian
Volcanic glass; chiefly rhyolitic silica glass (SiO₂-rich, commonly ~70–80% SiO₂)
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; black to dark brown, vitreous luster, noncrystalline/amorphous structure, and no cleavage. The sharp, strongly curved conchoidal fractures and glassy surfaces are characteristic; this specimen may be naturally weathered or knapped.
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Physical properties
Hardness about 5–5.5 Mohs; black to dark brown, vitreous luster, noncrystalline/amorphous structure, and no cleavage. The sharp, strongly curved conchoidal fractures and glassy surfaces are characteristic; this specimen may be naturally weathered or knapped.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically at volcanic domes and flow margins. It is generally geologically young, ranging from recent to Pleistocene or older volcanic episodes.
Uses & applications
Used historically for razor-sharp knives, arrowheads, and surgical blades; today it is collected, carved, and occasionally used decoratively or in jewelry. Industrial use is limited because it fractures easily.
Geological facts
Obsidian can display sheen, snowflake patterns, mahogany bands, or rainbow colors from microscopic inclusions or flow structure. Its sharp edges can be sharper than many steel blades, though brittle.
Field identification & locations
Identify it by black glassy appearance, lack of visible crystals, conchoidal fracture, and a translucent brown edge under strong light; it is common in volcanic regions such as Mexico, the western United States, Iceland, and Armenia.
