igneous
Black obsidian
Volcanic glass; predominantly silica-rich, noncrystalline SiO₂
AI-generated identification · may be incorrectTypically black, vitreous, and strongly reflective with conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and low specific gravity around 2.3–2.6. The smoothly rounded shape indicates tumbling or polishing.
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Physical properties
Typically black, vitreous, and strongly reflective with conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and low specific gravity around 2.3–2.6. The smoothly rounded shape indicates tumbling or polishing.
Formation & geological history
Forms when rhyolitic lava cools so rapidly that crystals cannot develop, generally in Quaternary to Cenozoic volcanic environments. Common sources include Mexico, the western United States, Iceland, Armenia, and Japan.
Uses & applications
Used for ornamental carvings, cabochons, beads, polished stones, and historically razor-sharp tools and arrowheads. It has collector value but is generally not a precious gemstone.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks a crystal lattice. Flow bands, snowflake patterns, or translucent brown/gray zones may indicate varieties such as snowflake or mahogany obsidian.
Field identification & locations
Identify it by its uniform glassy black appearance, sharp conchoidal edges, and lack of visible grains or crystals; it scratches glass but is softer than quartz. Avoid confusing it with black glass, basalt, or slag, which may show bubbles or crystalline texture.
