igneous
Black Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like)
- Hardness
- 5-5
- Color
- Jet black to dark grey
- Luster
- Vitreous/glassy
Identified More igneous →
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like)
Formation & geological history
Formed when felsic lava rich in silica cools extremely rapidly, preventing the growth of crystals. Typically associated with Cenozoic to Holocene volcanic activity.
Uses & applications
Used in ornamental jewelry, scalpel blades for delicate surgery, and for spiritual/decorative carvings.
Geological facts
Unlike most rocks, obsidian is metastable and eventually devitrifies into quartz; it is technically a high-viscosity liquid that frozen in place. Ancient civilizations used it for razor-sharp tools.
Field identification & locations
Identified by its extremely smooth, glassy surface and sharp, curved fracture lines. Commonly found in volcanic regions like the American West (Iceland, Oregon), Mexico, and Japan.
