igneous
Obsidian
Obsidian (volcanic glass), predominantly Silicon Dioxide (SiO2)
Hardness: 5-5.5 on Mohs scale; Color: typically black but can be brown, red (mahogany), or green; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- typically black but can be brown, red (mahogany), or green
- Luster
- vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically black but can be brown, red (mahogany), or green; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.35-2.6
Formation & geological history
Formed from rapid cooling of felsic lava, preventing crystal growth. Usually found in margin of rhyolitic lava flows. Most geologically recent (rarely older than a few million years due to devitrification).
Uses & applications
Used for surgical scalpel blades, decorative jewelry, healing crystals, and historically for sharp weapons and tools (arrowheads).
Geological facts
Obsidian is technically a glass rather than a true mineral because it lacks a crystalline structure. It was highly prized in pre-Columbian Mesoamerica for ritual knives and mirrors.
Field identification & locations
Identify by its extremely smooth, glassy appearance and sharp, curved (conchoidal) edges when broken. Found in volcanic regions like the American West (Glass Buttes, OR), Iceland, and Mexico.
