igneous
Obsidian
Volcanic Glass (SiO2 dominant)
Hardness: 5-5.5 (Mohs scale); Color: Black, dark green, or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black, dark green, or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, dark green, or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. It typically originates from volcanic margins or lava flows during the Cenozoic era to recent times.
Uses & applications
Used in gemstone jewelry, ornamental carvings, high-end surgical scalpel blades due to its ability to hold an edge sharper than steel, and as an abrasive in industry.
Geological facts
Because it is chemically unstable over long geological time periods, obsidian older than several million years is rare as it eventually devitrifies (crystallizes) into quartz and feldspar.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture pattern and extremely sharp edges. Found in volcanic regions like Glass Buttes (Oregon), Yellowstone, or the Aeolian Islands.
