igneous
Obsidian (likely black obsidian)
Volcanic glass; predominantly rhyolitic silica (SiO₂-rich), noncrystalline
AI-generated identification · may be incorrectUsually black with vitreous to glossy luster, conchoidal fracture, and sharp irregular edges; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The jagged, shiny form appears shaped or chipped, but the image alone cannot exclude industrial glass or slag.
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Physical properties
Usually black with vitreous to glossy luster, conchoidal fracture, and sharp irregular edges; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The jagged, shiny form appears shaped or chipped, but the image alone cannot exclude industrial glass or slag.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during volcanic eruptions in the Quaternary and older volcanic periods. Major sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for ornamental carvings, jewelry, tools, and historically razor-sharp blades; the distinctive flake shape is also popular in collecting. Commercial value is generally modest unless unusually patterned or exceptionally crafted.
Geological facts
Obsidian is technically volcanic glass rather than a crystalline mineral. Its conchoidal fracture can produce edges sharper than surgical steel, though they are brittle.
Field identification & locations
Identify it by glassy black appearance, very sharp curved fracture surfaces, lack of visible grains, and inability to scratch it easily with a steel knife. Common specimens sell for about $5–$30; shaped or larger display pieces commonly range from $20–$100.
