igneous
Polished black obsidian
Volcanic glass; predominantly rhyolitic silica (SiO₂), with minor iron, magnesium, and alkalis
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; opaque black, vitreous/glassy luster, conchoidal fracture, no cleavage, and typical specific gravity around 2.35–2.60. The specimen appears polished and shaped, so natural edges and texture are obscured.
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Physical properties
Hardness about 5–5.5 Mohs; opaque black, vitreous/glassy luster, conchoidal fracture, no cleavage, and typical specific gravity around 2.35–2.60. The specimen appears polished and shaped, so natural edges and texture are obscured.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during Pleistocene to recent volcanic eruptions. Major sources include volcanic regions of the western United States, Mexico, Armenia, Iceland, and Japan.
Uses & applications
Used for jewelry, carvings, decorative objects, knife blades, and lapidary cabochons; historically fashioned into exceptionally sharp tools and arrowheads.
Geological facts
Obsidian is technically volcanic glass rather than a crystalline mineral. Hydration dating can estimate surface exposure age, though it is less reliable for heavily processed pieces.
Field identification & locations
Identify by its uniform glassy appearance, sharp conchoidal fracture, and lack of visible grains; black slag or manufactured glass may look similar. Common varieties include snowflake, mahogany, rainbow, and sheen obsidian.
