igneous
Black Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
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
- Color
- Jet black
- Luster
- Vitreous/Glassy
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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 from extrusive igneous activity when felsic lava cools rapidly with minimum crystal growth. It is relatively young geologically (usually Cenozoic) because it eventually devitrifies into quartz and feldspar crystals.
Uses & applications
Used primarily in jewelry and ornamental carvings. Historically used for extremely sharp cutting tools, blades, and arrowheads. Also used in modern medicine for high-precision surgical scalpel blades.
Geological facts
Obsidian is technically a glass, not a mineral, because it lacks an internal crystalline structure. It was highly prized by the Aztecs and Mayans for mirrors and sacrificial daggers.
Field identification & locations
Identify in the field by its characteristic conchoidal (shell-like) fracture patterns and sharp edges. It is commonly found in volcanic regions such as the American Northwest (Oregon, California), Mexico, Iceland, and Italy.
