igneous
Polished black obsidian
Volcanic glass; predominantly rhyolitic silica glass (approximately SiO₂-rich)
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; black to smoky blue-black, vitreous luster, conchoidal fracture, amorphous/non-crystalline structure, and no cleavage. The reflective surface and sharp curved fracture patterns are characteristic.
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Physical properties
Hardness about 5–5.5 Mohs; black to smoky blue-black, vitreous luster, conchoidal fracture, amorphous/non-crystalline structure, and no cleavage. The reflective surface and sharp curved fracture patterns are characteristic.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly during Quaternary volcanic eruptions. It is found in young volcanic flows and domes rather than as a deeply buried ancient rock.
Uses & applications
Used for cabochons, beads, carvings, knife replicas, ornaments, and lapidary collectibles; historically shaped into exceptionally sharp tools and projectile points.
Geological facts
Obsidian can display sheen, snowflake patterns, or mahogany-red bands from microscopic inclusions. Its edges can be sharper than surgical steel, though the material is brittle.
Field identification & locations
Identify it by glassy luster, smooth conchoidal surfaces, lack of visible crystals, and inability to scratch glass reliably in every specimen. Common localities include Iceland, Mexico, the western United States, Armenia, and Japan.
