igneous
Black Obsidian
Volcanic Glass (Silicon Dioxide with impurities)
Hardness: 5-5.5 (Mohs); Color: Jet black, sometimes translucent at edges; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black, sometimes translucent at edges
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black, sometimes translucent at edges; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Common in volcanic regions with recent activity (within a few million years).
Uses & applications
Historically used for arrowheads and blades; currently used in jewelry, decorative carvings, and scalpel blades for delicate surgeries due to its molecularly sharp edges.
Geological facts
Obsidian is technically a glass rather than a true mineral because it lacks a crystalline structure. It is metastable, meaning over geological time it eventually devitrifies into crystals.
Field identification & locations
Identified by its deep black color, smooth glassy texture, and sharp, shell-like (conchoidal) fractures. Often found in areas of recent rhyolitic volcanic activity like the American West or Iceland.
