igneous
Black Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black to dark grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich rhyolitic lava, which prevents crystal growth. It is found in geologically recent volcanic areas across all ages, though it devitrifies over millions of years.
Uses & applications
Used in gemstone jewelry, ornamental carvings, and specialized surgical scalpel blades due to its ability to produce an edge thinner and sharper than steel.
Geological facts
Obsidian fractures with extremely sharp edges, leading to its use by prehistoric people for arrowheads and knives. It is technically a mineraloid rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identifiable by its smooth, glassy appearance and distinctive conchoidal (shell-like) fractures. Common in volcanic regions such as the American West, Iceland, and Mexico.
