igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-6 (Mohs); Color: Black, dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.60
- Hardness
- 5-6 (Mohs)
- Color
- Black, dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Black, dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.60
Formation & geological history
Formed by the rapid cooling of silica-rich lava (rhyolitic) which prevents crystal growth. Usually found in geologically young volcanic areas (Cenozoic era).
Uses & applications
Used in surgical scalpels, jewelry (beads, cabochons), ornamental carvings, and historically for sharp tools like arrowheads and knives.
Geological facts
Obsidian fractures with an edge that is thinner and sharper than a high-quality steel surgical scalpel. It is technically a mineraloid rather than a mineral because it lacks a crystal structure.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture and sharp edges. Common in volcanic regions of Iceland, Mexico, Italy, and the western United States.
