igneous
Black Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 (Mohs scale); Color: Jet black to dark brownish-black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6.
- Hardness
- 5-5
- Color
- Jet black to dark brownish-black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Jet black to dark brownish-black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6.
Formation & geological history
Formed when silica-rich lava cools so rapidly that crystals do not have time to grow. It is extrusive igneous rock, typically found in areas with geologically recent volcanic activity (Cenozoic era).
Uses & applications
Historically used for arrowheads and blades; currently used in jewelry (beads and cabochons), ornamental carvings, and specialized surgical scalpels due to its ability to produce an edge thinner than steel.
Geological facts
Because it is a glass rather than a true mineral, it is chemically unstable over millions of years and eventually devitrifies (turns into rock). It can produce a cutting edge that is molecularly thin.
Field identification & locations
Identify by its glassy texture, deep black color, and distinctive shell-like (conchoidal) fracture patterns. Found commonly in volcanic regions like the American West (Oregon, California, Iceland, and Mexico).
