igneous
Likely obsidian (rough fragment)
Volcanic glass, predominantly silica-rich rhyolitic composition (SiO₂-rich, noncrystalline)
AI-generated identification · may be incorrectBlack to dark gray, vitreous luster, conchoidal fracture, no visible crystal structure, and Mohs hardness about 5–5.5; sharp edges are typical. The photograph is not sufficient to distinguish obsidian confidently from dark slag or another glassy volcanic rock.
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Physical properties
Black to dark gray, vitreous luster, conchoidal fracture, no visible crystal structure, and Mohs hardness about 5–5.5; sharp edges are typical. The photograph is not sufficient to distinguish obsidian confidently from dark slag or another glassy volcanic rock.
Formation & geological history
Forms when silica-rich lava cools extremely rapidly, preventing crystal growth, mainly during Quaternary to older volcanic eruptions. Common in volcanic regions such as the western United States, Mexico, Iceland, Armenia, and Japan.
Uses & applications
Used historically for knives, arrowheads, and ritual tools; today it is used for carvings, jewelry, decorative objects, and geological collections. It has limited construction value.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Weathering may produce a dull surface or hydration rind, while snowflake obsidian contains radiating cristobalite inclusions.
Field identification & locations
Identify by its glassy appearance, conchoidal fracture, sharp margins, and lack of cleavage; a streak test is usually absent or pale. A scratch test, magnification, and locality information would help rule out industrial glass or slag.
