igneous
Obsidian
Obsidian (principally SiO2 + MgO, Fe3O4)
Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes with gray or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black, sometimes with gray or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes with gray or brown; 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 (felsic) lava, which prevents crystal growth. Most specimens observed are from the Cenozoic era or younger as it devitrifies over time.
Uses & applications
Used historically for sharp tools and weapons (scalpels, arrowheads). Modern uses include jewelry, decorative carvings, and specialized surgical blades.
Geological facts
Obsidian is technically a natural volcanic glass rather than a true mineral because it lacks a crystalline structure. It was highly prized in the Stone Age for its razor-sharp edges.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture and extremely smooth, glassy surface. Commonly found in volcanic regions like the American West, Mexico, Iceland, and Italy.
