igneous
Obsidian (likely black volcanic glass)
Obsidian; silica-rich, noncrystalline volcanic glass, typically rhyolitic (approximately 70–75% SiO₂)
AI-generated identification · may be incorrectUsually jet-black with a vitreous luster, conchoidal fracture, and sharp glassy edges; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.60. The photographed piece may also be slag or another black glass, which cannot be excluded visually.
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Physical properties
Usually jet-black with a vitreous luster, conchoidal fracture, and sharp glassy edges; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.60. The photographed piece may also be slag or another black glass, which cannot be excluded visually.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals do not develop, generally in volcanic flows, domes, and margins; most specimens are Quaternary to Cenozoic, though older deposits occur.
Uses & applications
Used for ornamental objects, lapidary pieces, jewelry, arrowheads, and historically cutting tools; its sharp fracture can produce exceptionally fine edges.
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 trace elements.
Field identification & locations
Identify by its glassy surface, curved conchoidal fractures, sharp margins, and lack of visible grains; common localities include Mexico, the western United States, Armenia, Japan, and Iceland. A streak, density, and close inspection are useful for distinguishing it from industrial slag.
