igneous
Obsidian
Volcanic Glass (Amorphous Silica)
Hardness: 5-5.5 (Mohs scale); Color: Black to dark green/brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black to dark green/brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black to dark green/brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (rhyolitic) lava, preventing crystal growth. Usually found in volcanic regions of various ages, though it devitrifies over millions of years.
Uses & applications
Used for surgical scalpel blades, jewelry (cabochons), ornamental carvings, and historically for sharp tools/weapons like arrowheads.
Geological facts
Obsidian fractures with extremely sharp edges—thinner than a steel scalpel—which allows it to cut at a molecular level. It is technically a glass, not a mineral, because it lacks a crystalline structure.
Field identification & locations
Identify in the field by its characteristic conchoidal (shell-like) fracture patterns and glassy look. Commonly found in locations like the Western United States, Mexico, and Iceland.
