igneous
Likely obsidian fragment
Volcanic glass, predominantly rhyolitic composition (SiO₂-rich)
AI-generated identification · may be incorrectDark brown to black, vitreous luster, amorphous rather than crystalline, with sharp conchoidal fractures; Mohs hardness about 5–5.5 and no cleavage. The glossy surfaces and curved fracture edges are characteristic, though the piece may be polished or naturally chipped.
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Physical properties
Dark brown to black, vitreous luster, amorphous rather than crystalline, with sharp conchoidal fractures; Mohs hardness about 5–5.5 and no cleavage. The glossy surfaces and curved fracture edges are characteristic, though the piece may be polished or naturally chipped.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth; most examples are Quaternary to recent, though older deposits occur. Common sources include volcanic regions of the western United States, Mexico, Armenia, Iceland, and Japan.
Uses & applications
Historically used for knives, arrowheads, and cutting tools because it fractures to extremely sharp edges; today it is used for ornaments, carvings, jewelry, and geological collecting.
Geological facts
Obsidian is technically volcanic glass, not a conventional crystalline mineral. Brown varieties are often called mahogany obsidian when they contain reddish-brown iron-rich patches.
Field identification & locations
Identify it by its glassy sheen, very sharp curved fracture, lack of visible grains, and inability to scratch glass reliably in some pieces; distinguish it from slag by its uniform texture and natural conchoidal fracture. The photograph alone cannot confirm whether it is obsidian rather than dark glass or polished slag.
