igneous
Obsidian
Obsidian (Volcanic glass), SiO2 (70% or more)
Hardness: 5-5.5 (Mohs scale); Color: Typically black but can be green, brown, or gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Typically black but can be green, brown, or gray
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Typically black but can be green, brown, or gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich lava which prevents crystal growth. It is found in geologically young volcanic areas (Cenozoic era).
Uses & applications
Used for making surgical scalpel blades, jewelry (pendants, beads), ornamental carvings, and historically for sharp tools like arrowheads.
Geological facts
Obsidian fractures with extremely sharp edges—thinner and sharper than high-quality steel scalpels. It is not a true mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its distinct glassy luster and conchoidal (shell-like) fracture patterns. Found in volcanic regions such as the American Northwest, Iceland, and Mexico.
