igneous
Obsidian
Obsidian (Volcanic glass, primarily SiO2)
Hardness: 5 to 5.5 on the Mohs scale; Color: Black, dark brown, or grey; Luster: Vitreous (glassy); Crystal structure: Amorphous (structureless glass); Cleavage: None (conchoidal fracture); Specific gravity: 2.3 to 2.6.
- Hardness
- 5 to 5
- Color
- Black, dark brown, or grey
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale; Color: Black, dark brown, or grey; Luster: Vitreous (glassy); Crystal structure: Amorphous (structureless glass); Cleavage: None (conchoidal fracture); Specific gravity: 2.3 to 2.6.
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. It is relatively young geologically, as it eventually devitrifies or breaks down into other minerals over millions of years.
Uses & applications
Historically used for making sharp tools like arrows and blades; currently used in jewelry (as a gemstone), ornamental carvings, and specialized surgical scalpels due to its incredibly sharp edge.
Geological facts
Because it is a glass rather than a true mineral, it has no internal crystalline structure. Obsidian scalpels can have a cutting edge many times sharper than high-quality steel scalpels.
Field identification & locations
Identify by its distinctive glassy sheen and curved 'conchoidal' fractures that look like ripples or shells. Commonly found in volcanic regions such as the United States (Oregon, California), Mexico, Iceland, and Japan.
