igneous
Obsidian
Volcanic Glass (70% or more SiO2)
Hardness: 5-6 Mohs. Color: Black. Luster: Vitreous/Glassy. Structure: Amorphous (non-crystalline). Fracture: Conchoidal (shell-like curves). Specific Gravity: 2.35-2.6.
- Hardness
- 5-6 Mohs
- Color
- Black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 Mohs. Color: Black. Luster: Vitreous/Glassy. Structure: Amorphous (non-crystalline). Fracture: Conchoidal (shell-like curves). Specific Gravity: 2.35-2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. Typically found in volcanic areas from the Cenozoic era or younger.
Uses & applications
Historically used for surgical scalpels, arrowheads, and blades. Modern uses include jewelry (cabochons), ornamental carvings, and gemstone collection.
Geological facts
Because it's a glass, obsidian lacks an internal crystalline structure and is chemically unstable over geological time, eventually devitrifying into minerals like quartz and feldspar.
Field identification & locations
Identified by its jet-black color, sharp edges, and smooth, curved conchoidal fracture surfaces. Commonly found in Western USA, Mexico, Iceland, and Japan.
