igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 (Mohs); Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Black to dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed by the rapid cooling of silica-rich lava (rhyolitic) that prevents crystal growth. It is relatively young geologically as it eventually devitrifies into crystals.
Uses & applications
Used for making surgical scalpel blades, jewelry, decorative ornaments, and historically for sharp tools like arrowheads and knives.
Geological facts
Because it fractures with extremely sharp edges, obsidian was highly prized in the Stone Age. It is sometimes used today for heart surgery as it creates a cleaner cut than steel.
Field identification & locations
Identify by its smooth, glassy texture and sharp, curved (conchoidal) edges. Found in areas of recent volcanic activity such as the western USA, Mexico, and Iceland.
