igneous
Obsidian
Obsidian (Volcanic Glass, SiO2)
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes translucent on edges; Luster: Vitreous (glassy); Crystal structure: Amorphous; Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black, sometimes translucent on edges
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes translucent on edges; Luster: Vitreous (glassy); Crystal structure: Amorphous; Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. Usually found in volcanic regions from the Cenozoic era or younger due to its tendency to devitrify over time.
Uses & applications
Used historically for sharp tools like arrowheads and knives. Modern uses include surgical scalpel blades, jewelry, decorative ornaments, and semi-precious gemstones.
Geological facts
Because it has no crystal structure, obsidian can be fractured to a thickness of only a few molecules, making it sharper than even high-quality steel scalpels. It is technically a mineraloid, not a mineral.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture pattern and extremely smooth, glassy surface. Common in volcanic active or historically volcanic areas like Oregon (USA), Mexico, and Iceland.
