igneous
Polished obsidian
Volcanic glass, primarily silica-rich rhyolitic composition (approximately 70–75% SiO₂)
AI-generated identification · may be incorrectUsually black, brown, or mahogany with vitreous luster and conchoidal fracture; amorphous rather than crystalline, with Mohs hardness about 5–5.5 and specific gravity 2.35–2.65. The glossy surface and curved, shell-like breaks strongly fit polished obsidian.
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Physical properties
Usually black, brown, or mahogany with vitreous luster and conchoidal fracture; amorphous rather than crystalline, with Mohs hardness about 5–5.5 and specific gravity 2.35–2.65. The glossy surface and curved, shell-like breaks strongly fit polished obsidian.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, generally during relatively young volcanic eruptions from the Cenozoic to Recent. Common sources include Mexico, the western United States, Armenia, Iceland, and parts of Japan.
Uses & applications
Used for ornaments, cabochons, carvings, decorative objects, and historically extremely sharp blades and arrowheads. Its attractive sheen makes it a popular inexpensive lapidary material.
Geological facts
Obsidian is technically natural volcanic glass, not a true crystalline mineral. Flow bands, snowflake-like cristobalite inclusions, or golden/silver sheen can create decorative varieties.
Field identification & locations
Identify it by glassy luster, sharp conchoidal fracture, lack of visible grains, and inability to scratch glass reliably in some pieces; polished surfaces can obscure these traits. Estimated value: $10–$50 for a common polished piece, or $50–$200+ for large, unusual, or patterned specimens.
