igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5–6 Mohs; Color: Jet black with brownish streaks; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4
- Hardness
- 5–6 Mohs
- Color
- Jet black with brownish streaks
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–6 Mohs; Color: Jet black with brownish streaks; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava which prevents crystal growth. Usually found in margin of rhyolitic lava flows. Most specimens are relatively young geologically (Cenozoic) because glass eventually devitrifies into crystals.
Uses & applications
Historically used for arrowheads and blades; currently used in jewelry (lapidary), as a gemstone, and in high-end surgical scalpel blades because it can be fractured to a molecularly thin edge.
Geological facts
Because it is a glass and not a mineral, it has no internal crystal structure. Mahogany obsidian (like this appears to be) contains inclusions of hematite or magnetite creating brown-to-red swirls.
Field identification & locations
Identify by its glass-like texture and sharp, curved (conchoidal) edges. Often found near volcanic regions like Glass Buttes, Oregon or the Yellowstone area. Collectors should look for unique banding and clarity.
