igneous
Obsidian (likely black obsidian)
Volcanic glass; primarily SiO₂-rich rhyolitic composition
AI-generated identification · may be incorrectUsually black, vitreous, and noncrystalline, with conchoidal fracture and Mohs hardness about 5–5.5; no cleavage. The glossy, smoothly shaped surface suggests a polished, tumbled, or water-worn piece, though the photo alone cannot confirm identity.
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Physical properties
Usually black, vitreous, and noncrystalline, with conchoidal fracture and Mohs hardness about 5–5.5; no cleavage. The glossy, smoothly shaped surface suggests a polished, tumbled, or water-worn piece, though the photo alone cannot confirm identity.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary volcanic activity and older eruptions. Common sources include Mexico, the western United States, Armenia, Turkey, and Iceland.
Uses & applications
Used historically for exceptionally sharp blades and arrowheads; today it is collected, carved, polished, and occasionally used in jewelry and decorative objects.
Geological facts
Obsidian can be jet-black, mahogany, snowflake-patterned, rainbow, or sheen-bearing. It is technically a mineraloid rather than a true mineral because it lacks an organized crystal lattice.
Field identification & locations
Identify it by glassy luster, uniform dark color, very sharp conchoidal edges, and lack of visible grains; test cautiously because fresh edges can cut skin. Similar black slag or glass may require a streak, density, or provenance test.
