igneous
Obsidian
Volcanic glass (SiO2 - Silicon Dioxide with impurities)
Hardness: 5–6 Mohs; Color: Jet black to dark green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6.
- Hardness
- 5–6 Mohs
- Color
- Jet black to dark green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–6 Mohs; Color: Jet black to dark green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6.
Formation & geological history
Formed from extrusive igneous activity when felsic lava (rich in silica) cools so rapidly that crystal growth is inhibited. It often forms at the edges of basaltic lava flows or when lava enters water.
Uses & applications
Historically used for blades, arrowheads, and mirrors. Modern uses include surgical scalpels, ornamental jewelry, carvings, and feng shui 'protective' stones.
Geological facts
Because it has no crystal structure, its edges can be thinned down to almost molecular thinness, making it sharper than high-quality steel scalpels. Famous locations include the Glass Buttes in Oregon and various sites across the Ring of Fire.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture, extremely smooth surface, and razor-sharp edges if broken. Common in Cenozoic volcanic regions; look for it in areas with rhyolitic eruptions.
