igneous
Obsidian
Volcanic glass (silica-rich rhyolitic melt; primarily SiO₂)
AI-generated identification · may be incorrectTypically black, opaque to translucent at thin edges, with a vitreous luster and conchoidal fracture; hardness about 5–5.5 Mohs, no crystal structure or cleavage, and specific gravity around 2.35–2.65. The glossy, glass-like surface and sharp curved fractures fit obsidian, though the specimen may be weathered or natura…
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Physical properties
Typically black, opaque to translucent at thin edges, with a vitreous luster and conchoidal fracture; hardness about 5–5.5 Mohs, no crystal structure or cleavage, and specific gravity around 2.35–2.65. The glossy, glass-like surface and sharp curved fractures fit obsidian, though the specimen may be weathered or naturally rough.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly in volcanic domes and lava flows. Most specimens are Cenozoic or younger, though obsidian occurs wherever suitable volcanic eruptions occurred.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads; today it is collected, carved, polished for ornament, and occasionally used in jewelry. It has limited construction value compared with ordinary volcanic rock.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Black color commonly results from iron and other impurities; flow bands, snowflake patterns, or sheen varieties may occur.
Field identification & locations
Identify it by its glassy appearance, conchoidal fracture, sharp edges, and inability to scratch glass reliably in every case; distinguish it from slag by its natural flow texture and lack of abundant bubbles. Common sources include Mexico, the western United States, Iceland, Armenia, and Japan.
