igneous
Black Obsidian
Obsidian (Amorphous volcanic glass, SiO2)
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3-2.6
- Hardness
- 5-5
- Color
- Black, sometimes dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. It is relatively young in geological terms because it eventually breaks down or devitrifies into crystals.
Uses & applications
Used for making surgical scalpels (sharper than steel), omamental jewelry, gemstone carvings, and historical tools like arrowheads and knives.
Geological facts
Obsidian can be fractured to create an edge thinner and sharper than a surgeon's steel scalpel. The black color usually comes from microscopic inclusions of magnetite or hematite.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture and sharp edges. It is common in volcanic regions like the American West (Iceland, Mexico, Japan). Collectors should look for smooth, glassy surfaces.
