igneous
Obsidian
Volcanic Glass (70-75% SiO2)
Hardness: 5-5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Forms from the rapid cooling of felsic (silica-rich) lava, preventing crystal growth. Usually found in volcanic margins of the Cenozoic or younger eras as it devitrifies over time.
Uses & applications
Used for surgical scalpel blades, jewelry (cabochons), talismans, and ornamental carvings. Historically used for arrowheads and knives.
Geological facts
Because it has no crystal structure, its edges can reach near-molecular thinness, making it sharper than even high-quality steel scalpels. Exposure to water over millions of years causes it to turn into perlite.
Field identification & locations
Identify via its smooth, glassy texture and curved, shell-like (conchoidal) fractures. Found in areas with geologically recent volcanic activity like Oregon, California, and Iceland.
