igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5; Color: Black to dark brown; Luster: Vitreous/glassy; Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black to dark brown
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-5.5; Color: Black to dark brown; Luster: Vitreous/glassy; Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Forms from the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. Most specimens are relatively young geologically (less than 20 million years) because the glass eventually devitrifies or breaks down.
Uses & applications
Used historically for sharp tools and weapons like arrowheads; modern uses include scalpel blades for surgery, jewelry, and ornamental carvings.
Geological facts
Obsidian is naturally occurring volcanic glass. Despite its black appearance, it is often transparent when sliced thin. It breaks with extremely sharp edges, sometimes down to nearly molecular thickness.
Field identification & locations
Identify by its smooth, glassy texture and distinctive conchoidal (shell-like) fracture patterns. Found in volcanic regions like the American West (Iceland, Mexico, Japan).
