igneous
Obsidian (black volcanic glass), possibly slag or basalt
Obsidian; chiefly amorphous volcanic silica glass, approximately SiO₂-rich rhyolitic composition
AI-generated identification · may be incorrectBlack, very fine-grained to glassy, with vitreous-to-dull luster and conchoidal-looking fractures; Mohs hardness about 5–5.5, no cleavage, and relatively low specific gravity around 2.3–2.6. The image alone cannot exclude industrial slag or dark basalt.
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Physical properties
Black, very fine-grained to glassy, with vitreous-to-dull luster and conchoidal-looking fractures; Mohs hardness about 5–5.5, no cleavage, and relatively low specific gravity around 2.3–2.6. The image alone cannot exclude industrial slag or dark basalt.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, usually during Quaternary to older volcanic eruptions. Common sources include volcanic regions of the western United States, Mexico, Iceland, Italy, and Armenia.
Uses & applications
Used for ornamental objects, cabochons, tools, and historically sharp blades; quality snowflake, rainbow, or sheen varieties are used in jewelry. It has little construction value compared with basalt.
Geological facts
Obsidian can produce exceptionally sharp edges because it fractures conchoidally. Unlike crystalline quartz, it is technically a mineraloid rather than a true mineral.
Field identification & locations
Look for a glassy surface, curved shell-like fracture, sharp edges, and occasional flow bands or inclusions; a streak test is usually white or colorless. Slag commonly shows bubbles, metallic flecks, or irregular melted textures, while basalt is visibly crystalline under magnification.
