igneous
Obsidian
Volcanic Glass (SiO2 rich)
Hardness: 5-5.5 on Mohs scale; Color: typically black but can be gray; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none, but exhibits conchoidal fracture.
- Hardness
- 5-5
- Color
- typically black but can be gray
- Luster
- vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically black but can be gray; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none, but exhibits conchoidal fracture.
Formation & geological history
Formed by the rapid cooling of silica-rich lava (rhyolitic) that prevents crystal growth. It is relatively young geologically because it eventually devitrifies into crystals over time.
Uses & applications
Used historically for sharp tools and weapons; currently used in scalpels for surgery, jewelry, decorative ornaments, and metaphysical collecting.
Geological facts
Obsidian fractures into edges thinner than a human hair. It is not a true mineral because it lacks a crystalline structure; it is technically a mineraloid or volcanic glass.
Field identification & locations
Identify in the field by its distinctive conchoidal (shell-like) fracture patterns and extremely smooth, glassy surface. Commonly found in volcanic regions like the American West (Glass Buttes, Oregon), Mexico, and Iceland.
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