igneous
Black Obsidian
Volcanic Glass (predominantly SiO2)
Hardness: 5-5.5 (Mohs scale); Color: Jet black, can be translucent on edges; Luster: Vitreous (glassy); Crystal structure: Amorphous; Cleavage: None (conchoidal fracture); SG: 2.4
- Hardness
- 5-5
- Color
- Jet black, can be translucent on edges
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Jet black, can be translucent on edges; Luster: Vitreous (glassy); Crystal structure: Amorphous; Cleavage: None (conchoidal fracture); SG: 2.4
Formation & geological history
Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Found in geologically young volcanic areas (Cenozoic era).
Uses & applications
Used for surgical scalpel blades, jewelry (beads, cabochons), ornamental carvings, and historical tool making (arrowheads).
Geological facts
Obsidian is technically a mineraloid, not a mineral, because it lacks a crystalline structure. Ancient Aztecs used obsidian mirrors for divination.
Field identification & locations
Identify by its glass-like appearance and sharp, curved conchoidal fracture edges. Found in volcanic regions like Iceland, Mexico, and the Western United States.
