igneous
Obsidian
Volcanic glass
Hardness: 5-6 (Mohs); Color: Black, sometimes green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.3-2.6
- Hardness
- 5-6 (Mohs)
- Color
- Black, sometimes green or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Black, sometimes green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.3-2.6
Formation & geological history
Formed from the rapid cooling of silica-rich (rhyolitic) lava, which prevents the growth of mineral crystals. Typically found in Cenozoic volcanic regions.
Uses & applications
Used for making surgical scalpels due to its sharper-than-steel edge, fashioning jewelry, arrowheads/tools historically, and for ornamental decor.
Geological facts
Obsidian fractures with extremely sharp edges, giving it a history of use as knives and weapons. It is technically a mineraloid because it lacks a crystalline structure.
Field identification & locations
Identify by its characteristic glassy luster and curved, shell-like (conchoidal) fractures. Found in volcanic areas like the Western USA, Iceland, and Mexico.
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