igneous
Obsidian
Extrusive igneous volcanic glass
Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown; Luster: Vitreous (glassy); Crystal structure: Amorphous; Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- Black to dark brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown; Luster: Vitreous (glassy); Crystal structure: Amorphous; 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 of geologically recent volcanic activity.
Uses & applications
Used in ancient times for cutting tools; currently used in jewelry (gemstones), decorative art, and specialized surgical scalpel blades.
Geological facts
Obsidian is technically not a mineral because it is a glass and does not have a crystalline structure. It was highly prized by Aztecs and Mayans for weaponry.
Field identification & locations
Identified by its sharp, curved (conchoidal) fracture surfaces and intense glassy shine. Found in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.
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