igneous
Black Obsidian
Volcanic Glass (Primary composition: SiO2 - Silicon Dioxide)
Hardness: 5.0 - 6.0 on Mohs scale; Color: Jet black to dark grayish; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35 - 2.6
- Hardness
- 5
- Color
- Jet black to dark grayish
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5.0 - 6.0 on Mohs scale; Color: Jet black to dark grayish; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35 - 2.6
Formation & geological history
Formed from rapid cooling of high-silica felsic lava (rhyolitic) which prevents crystal growth. It is found in volcanic regions of various ages, particularly the Cenozoic era.
Uses & applications
Used in gemstone jewelry (cabochons), ornamental decorative items, and historically for ultra-sharp surgical blades and prehistoric tools like spearheads.
Geological facts
Because it is a glass rather than a true mineral, it is technically a 'mineraloid'. It breaks with extremely sharp edges—thinner than a steel scalpel—and was favored by the Maya and Aztecs for weaponry.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture and smooth, glassy surface. Commonly found in locations like Yellowstone (USA), Iceland, Mexico, and Japan.
