igneous
Black Obsidian
Volcanic Glass (Amorphous Silicon Dioxide)
Hardness: 5-6 (Mohs); Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-6 (Mohs)
- Color
- Jet black
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Obsidian forms when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is typically found in geological areas that have experienced rhyolitic eruptions within the last few million years.
Uses & applications
Historically used for arrowheads and blades; currently used in jewelry, art, and high-quality surgical scalpel blades.
Geological facts
Because it is a glass and lacks a crystalline structure, obsidian edges can reach near-molecular thinness, making them sharper than steel scalpels.
Field identification & locations
Identify by its distinctive shell-like (conchoidal) fracture patterns and extremely sharp edges. Commonly found in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.
