igneous
Obsidian
Amorphous volcanic glass (SiO2)
Hardness: 5.0-6.0; Color: Black, dark green, or brownish; Luster: Vitreous; Crystal Structure: Amorphous/None; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5
- Color
- Black, dark green, or brownish
- Luster
- Vitreous
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Physical properties
Hardness: 5.0-6.0; Color: Black, dark green, or brownish; Luster: Vitreous; Crystal Structure: Amorphous/None; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from the rapid cooling of silica-rich lava, preventing crystal growth. Most specimens are relatively young geologically (Cenozoic) because the glass eventually devitrifies.
Uses & applications
Historically used for arrowheads and scrapers. Modern uses include surgical scalpel blades, jewelry, and ornamental decor.
Geological facts
Obsidian fractures into edges that are many times sharper than high-quality steel scalpels. It is technically a mineraloid rather than a true mineral because it lacks a crystalline structure.
Field identification & locations
Identified by its glassy surface and conchoidal (shell-like) fracture pattern. Found in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.
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