igneous
Black Obsidian
Obsidian (Volcanic Glass, ~70% SiO2)
Hardness: 5-5.5; Color: Jet black to dark gray; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- Jet black to dark gray
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5; Color: Jet black to dark gray; 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 where high-silica lava cools extremely rapidly, preventing crystal growth. Usually associated with geological ages from the Cenozoic to recent due to its tendency to devitrify over time.
Uses & applications
Used for making extremely sharp surgical scalpels, decorative jewelry (beads, cabochons), and ornamental carvings. Historically used for arrowheads and scrapers.
Geological facts
Because it has no crystal structure, its edges can reach almost molecular thinness, making it sharper than high-quality steel scalpels. It is technically a mineraloid rather than a mineral.
Field identification & locations
Identify by its glass-like appearance, smooth curved (conchoidal) fracture lines, and sharp edges. Common in volcanic regions such as Iceland, Mexico, and the western United States (e.g., Oregon, California).
