igneous
Obsidian
Obsidian (Amorphous volcanic glass)
Hardness: 5-6 (Mohs); Color: Black (sometimes with brown or sheen); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3-2.6
- Hardness
- 5-6 (Mohs)
- Color
- Black (sometimes with brown or sheen)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Black (sometimes with brown or sheen); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3-2.6
Formation & geological history
Formed when extrusive felsic lava cools rapidly with minimal crystal growth. Often found in geologically young volcanic areas (Quaternary age or later) where silica-rich lava hits water or air.
Uses & applications
Used in gemstone jewelry, ornamental carvings, high-end surgical scalpels for smooth incisions, and historical production of sharp tools and weapons.
Geological facts
Because it is glass and not a true mineral, it is technically unstable over millions of years and will eventually devitrify into a crystalline rock. Ancient civilizations used it as a trade commodity similar to currency.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture which creates razor-sharp edges. Commonly found in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.
