igneous
Obsidian
Obsidian (principally SiO2)
Hardness: 5-6 (Mohs scale); Color: Black, sometimes dark brown or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-6 (Mohs scale)
- Color
- Black, sometimes dark brown or green
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Black, sometimes dark brown or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed from rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. It is relatively young geologically (mostly Cenozoic) because it eventually devitrifies into crystalline rock over millions of years.
Uses & applications
Used for surgical scalpel blades, jewelry (cabochons and beads), ornamental carvings, and ancient tools like arrowheads and knives.
Geological facts
Because it heals to a molecularly thin edge, obsidian scalpels are sharper than high-quality steel. It was highly prized by ancient civilizations like the Aztecs and Greeks for tools and mirrors.
Field identification & locations
Identify in the field by its characteristic curved (conchoidal) fracture lines and glassy texture. Commonly found in volcanic regions such as the American Northwest, Mexico, and Iceland. Collectors should look for 'flow' patterns or 'sheen'.
