igneous
Obsidian (Black)
Obsidian (approx. 70–75% SiO2)
Hardness: 5–6 Mohs Scale; Color: Jet black; Luster: Vitreous/Glossy; Structure: Amorphous (glassy, no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5–6 Mohs Scale
- Color
- Jet black
- Luster
- Vitreous/Glossy
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Physical properties
Hardness: 5–6 Mohs Scale; Color: Jet black; Luster: Vitreous/Glossy; Structure: Amorphous (glassy, no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Common in areas of geologically recent volcanic activity (Cenozoic era).
Uses & applications
Used historically for sharp cutting tools/scalpels; currently used in jewelry (cabochons), ornamental carvings, and for meditation/metaphysical collections.
Geological facts
Because it is a glass and not a crystal, obsidian is metastable and will eventually devitrify into quartz and feldspar over millions of years. This is why few obsidian specimens are older than the Cretaceous period.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture and sharp edges. Common in volcanic regions like the American West (Glass Buttes, OR), Iceland, and Mexico. Collectors should look for a uniform, glass-like transparency on thin edges.
