igneous
Obsidian
Volcanic Glass (Amorphous Silica)
Hardness: 5-5.5 (Mohs scale), Color: Jet black, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Common in areas with recent volcanic activity like the western US, Mexico, and Iceland. Most found is from the Cenozoic era.
Uses & applications
Used for making surgical scalpels for high-precision cutting, obsidian mirrors, jewelry (cabochons), and ornamental carvings. Historically used for arrowheads and blades.
Geological facts
Because it has no crystal structure, its fracture edges can reach almost molecular thinness, making it sharper than high-quality steel scalpels. It is technically a mineraloid rather than a true mineral.
Field identification & locations
Identify in the field by its characteristic smooth, curved conchoidal (shell-like) fracture edges and intense glassy luster. Often found in volcanic flows or glassy domes. Found commonly in Oregon, California, and Mexico.
