igneous
Black Obsidian
Obsidian (Volcanic Glass, SiO2 + impurities)
Hardness: 5-5.5 (Mohs scale); Color: Jet black to dark brown; Luster: Vitreous; Crystal structure: Amorphous; Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.60
- Hardness
- 5-5
- Color
- Jet black to dark brown
- Luster
- Vitreous
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Jet black to dark brown; Luster: Vitreous; Crystal structure: Amorphous; Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.60
Formation & geological history
Obsidian is an extrusive volcanic rock that forms when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is most commonly found in areas with recent volcanic activity (Cenozoic era).
Uses & applications
Historically used for sharp tools like arrowheads and blades; currently used for scalpel blades in heart surgery, semi-precious jewelry, home decor, and metaphysical collecting.
Geological facts
Because it is glass and not a crystal, obsidian is technically a mineraloid rather than a mineral. It was once highly prized by ancient civilizations for its ability to be knapped into the sharpest edge known to man.
Field identification & locations
Identified in the field by its dark, glassy appearance and characteristic conchoidal (shell-like) fractures. Common locations include the American West (Iceland, Oregon, California) and Mexico.
