igneous
Obsidian
Volcanic glass (SiO2)
Hardness: 5-6 Mohs. Color: Jet black to dark brown. Luster: Vitreous/glassy. Crystal structure: Amorphous. Cleavage: None (conchoidal fracture). Specific gravity: 2.35-2.6.
- Hardness
- 5-6 Mohs
- Color
- Jet black to dark brown
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-6 Mohs. Color: Jet black to dark brown. Luster: Vitreous/glassy. Crystal structure: Amorphous. Cleavage: None (conchoidal fracture). Specific gravity: 2.35-2.6.
Formation & geological history
Formed by the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. Often found in volcanic areas from the Cenozoic era.
Uses & applications
Used for making extremely sharp surgical scalpels, decorative jewelry, and ornamental carvings. Historically used for arrowheads and scrapers.
Geological facts
Unlike most rocks, obsidian is not made of crystals; it is a true volcanic glass. It breaks with extremely sharp edges that can be even finer than a steel blade.
Field identification & locations
Identify in the field by its distinct glassy luster and conchoidal (shell-like) fracture patterns. Commonly found in areas with recent volcanic activity like Oregon, Iceland, or Mexico.
