igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5–6 on the Mohs scale; Color: typically jet black, can be grey, green, or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.
- Hardness
- 5–6 on the Mohs scale
- Color
- typically jet black, can be grey, green, or brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–6 on the Mohs scale; Color: typically jet black, can be grey, green, or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.
Formation & geological history
Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in volcanic regions where the cooling occurred quickly, often at the edges of a lava flow or where lava contacted water or air.
Uses & applications
Historically used for making sharp tools like arrowheads and blades; currently used in jewelry (gemstones), ornamental carvings, and specialized surgical scalpels due to its incredibly sharp edge.
Geological facts
Because it lacks a crystalline structure, obsidian edges can reach nearly molecular thinness, making them sharper than steel scalpels. It was highly prized by ancient civilizations like the Aztecs and Mayans.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and glassy luster. It is common in areas with recent volcanic activity like the American Northwest, Iceland, Mexico, and Japan.
