igneous
Obsidian
Vitreous volcanic silicate (70-75% SiO2)
Hardness: 5-5.5 (Mohs); Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
Formation & geological history
Formed by the rapid cooling of silica-rich (rhyolitic) lava, preventing crystal growth. It typically dates to the Cenozoic era as it eventually devitrifies.
Uses & applications
Historically used for sharp tools (blades/arrows). Modern uses include high-quality surgical scalpels, ornamental jewelry, and decorative gemstones.
Geological facts
Unlike most rocks, obsidian is naturally occurring volcanic glass. It is so sharp that it can be fractured to a cutting edge only a few molecules thick.
Field identification & locations
Identifiable by its sharp, curved (conchoidal) fracture lines and smooth, glassy surface. Commonly found in volcanic regions such as Iceland, Mexico, and the Western USA (Oregon/California).
