igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5; Color: Jet black, dark brown, or gray; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black, dark brown, or gray
- Luster
- Vitreous/glassy
Identified More igneous →
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Physical properties
Hardness: 5-5.5; Color: Jet black, dark brown, or gray; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Common in young volcanic areas (Cenozoic era) because it becomes unstable over millions of years.
Uses & applications
Used for making surgical scalpel blades, ornamental jewelry (beads, cabochons), and mirrors. Historically used for weapons like arrowheads and knives.
Geological facts
Obsidian is technically a supercooled liquid rather than a true mineral because it lacks a crystalline structure. It is one of the sharpest materials known to science when fractured.
Field identification & locations
Identify by its smooth, glassy surface and distinctive curved (conchoidal) fractures. Found in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.
