igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous/glassy, Crystal structure: Amorphous, Cleavage: None (conchoidal fracture), Specific gravity: 2.35-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous/glassy, Crystal structure: Amorphous, Cleavage: None (conchoidal fracture), Specific gravity: 2.35-2.6
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, where cooling occurs so quickly that crystals do not have time to grow. Most specimens are younger than 65 million years.
Uses & applications
Used in ancient times for cutting tools and spearheads; modern uses include surgical scalpel blades, jewelry, and ornamental decor.
Geological facts
Obsidian is actually a liquid that has become so viscous that it behaves like a solid. It is technically more of a glass than a true mineral because it lacks a crystal structure.
Field identification & locations
Identify by its smooth, shell-like (conchoidal) fracture and extremely sharp edges. Commonly found in volcanic regions like the American West, Mexico, Iceland, and Italy.
