igneous
Obsidian
Obsidian (volcanic glass), amorphous SiO2 + impurities
Hardness: 5-5.5 (Mohs scale); Color: Black, deep brown, or grey; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black, deep brown, or grey
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, deep brown, or grey; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed as an extrusive igneous rock through the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. It is relatively young geologically (mostly Cenozoic) because the glass eventually devitrifies into crystals.
Uses & applications
Used historically for sharp tools and weapons (scalpels, arrowheads); modern use in jewelry as a gemstone, decorative carvings, and in high-end surgical blades.
Geological facts
Because it breaks with a conchoidal fracture producing edges thinner than a steel scalpel, obsidian blades are still occasionally used in specialized cardiac and eye surgeries to minimize scarring.
Field identification & locations
Identify in the field by its smooth, glassy texture and distinctive curved, shell-like (conchoidal) fractures. Found in volcanic regions like the American Northwest, Mexico, Iceland, and Italy. Collectors look for 'sheen' varieties like gold or rainbow obsidian.
