igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 (Mohs scale); Color: Black, deep brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Black, deep brown, or grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, deep brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava which prevents crystal growth. It is common in areas with recent volcanic activity (Cenozoic era).
Uses & applications
Used in gemstone jewelry, ornamental carvings, and high-quality surgical scalpel blades due to its ability to hold an edge thinner than steel.
Geological facts
Obsidian fractures with extremely sharp edges, leading to its extensive historical use by prehistoric people for arrowheads, knives, and scrapers.
Field identification & locations
Identify by its glass-like texture and smooth, curved (conchoidal) fracture surfaces. Commonly found in volcanic regions like the American West, Mexico, and Iceland.
