igneous
Obsidian
Obsidian (Extrusive Volcanic Glass)
Hardness: 5-5.5 (Mohs scale); Color: Black with potential translucency at edges; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black with potential translucency at edges
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black with potential translucency at edges; 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 (rhyolitic) lava, which prevents crystals from growing. Usually found in geologically young areas with recent volcanic activity (Cenozoic era).
Uses & applications
Used for making exceptionally sharp surgical blades, high-end jewelry (cabochons), arrowheads/tools, and ornamental decorative stones.
Geological facts
Obsidian is naturally occurring volcanic glass. Its edges can be thinned down to a molecular level, making it many times sharper than a high-quality steel surgical scalpel. It is technically a 'mineraloid' because it lacks a crystal structure.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture, which produces curved ridges as seen in the photograph. It is commonly found in volcanic areas like the Western United States (Oregon, California), Iceland, and Mexico.
