igneous
Obsidian
Volcanic Glass (SiO2 + impurities)
Hardness: 5-5.5; Color: Jet black to dark green; Luster: Vitreous; Crystal structure: Amorphous; Cleavage: None (Conchoidal fracture); Specific gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Jet black to dark green
- Luster
- Vitreous
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Physical properties
Hardness: 5-5.5; Color: Jet black to dark green; Luster: Vitreous; Crystal structure: Amorphous; Cleavage: None (Conchoidal fracture); Specific gravity: 2.35-2.6
Formation & geological history
Formed by the rapid cooling of silica-rich felsic lava which prevents crystal growth. It is relatively young geologically (usually Cenozoic) as it eventually devitrifies into crystalline rock.
Uses & applications
Used for surgical scalpel blades, jewelry, decorative ornaments, and historically for arrowheads and cutting tools.
Geological facts
Obsidian fractures into edges that are thinner and sharper than the finest steel scalpels. Unlike most rocks, it is a glass with no internal crystalline structure.
Field identification & locations
Identifiable by its sharp, curved (conchoidal) fractures and shiny glass-like surface. Often found in volcanic regions such as the American Northwest, Iceland, and Mexico.
