igneous
Obsidian (likely snowflake-free black volcanic glass)
Volcanic glass; predominantly rhyolitic silica melt (SiO₂-rich, noncrystalline)
AI-generated identification · may be incorrectUsually glassy, black to dark brown, with vitreous luster and conchoidal fracture producing extremely sharp edges; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The flow-like surface textures and reflective fracture strongly fit obsidian, though the image alone cannot exclude industrial…
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Physical properties
Usually glassy, black to dark brown, with vitreous luster and conchoidal fracture producing extremely sharp edges; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The flow-like surface textures and reflective fracture strongly fit obsidian, though the image alone cannot exclude industrial glass or another volcanic rock.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary to older Cenozoic volcanic eruptions. Common sources include the western United States, Mexico, Armenia, Turkey, Italy, Iceland, and Japan.
Uses & applications
Used for ornamental carvings, tools, arrowheads, cabochons, polished specimens, and occasionally surgical blades; it has limited construction use.
Geological facts
Obsidian can preserve perlitic cracking and flow banding, and freshly fractured edges are sharper than most steel edges at microscopic scale. Hydration-rim dating can sometimes estimate its surface exposure age.
Field identification & locations
Identify by its uniform glassy appearance, conchoidal fracture, sharp edges, and lack of visible crystals; streak is generally absent or white. Check for bubbles, mold marks, or perfectly uniform artificial coloration if glass is suspected.
