igneous
Obsidian
Volcanic Glass (70% or more SiO2)
Hardness: 5-5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed from the rapid cooling of felsic lava, which prevents crystal growth. Common in areas with recent volcanic activity (Cenozoic era).
Uses & applications
Surgical scalpel blades, jewelry, decorative ornaments, and historical use for weaponry (arrowheads and knives).
Geological facts
Obsidian can be sharper than high-quality steel scalpels. It is technically a mineraloid rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its deep black color, glassy luster, and shell-like (conchoidal) fracture pattern. Found in volcanic regions like the American West, Iceland, and Italy.
