igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-6 (Mohs); Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3-2.6.
- 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: 2.3-2.6.
Formation & geological history
Formed when viscous lava from a volcano cools rapidly with minimal crystal growth. Usually found in margin of rhyolitic lava flows or along the edges of a volcanic dome. Most are less than few million years old.
Uses & applications
Historically used for arrowheads and knives. Today used in scalpel blades for surgery, jewelry, and as a decorative stone in landscaping and home decor.
Geological facts
Obsidian breaks with a conchoidal fracture, producing sharp edges that can be thinner and sharper than a steel scalpel. It was a highly prized trade item in pre-industrial societies.
Field identification & locations
Identify by its glass-like texture, lack of crystals, and sharp curved edges (conchoidal fracture). Common in volcanic regions such as the American West, Mexico, Iceland, and Italy.
