igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black to dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. It is relatively young geologically as it eventually devitrifies into crystals.
Uses & applications
Historically used for cutting tools, arrowheads, and mirrors. Today used in scalpel blades, jewelry, and as a collector's specimen.
Geological facts
Obsidian fractures with extremely sharp edges, sometimes thinner than a molecule. In ancient cultures, it was a highly traded commodity for making sharp weapons and tools.
Field identification & locations
Identify by its glassy texture and conchoidal (shell-like) fracture patterns. Frequently found in volcanic areas like the Western USA, Mexico, and Iceland.
