igneous
Obsidian
Volcanic Glass (Primary composition: SiO2 - Silicon Dioxide)
Hardness: 5-6 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-6 on Mohs scale
- Color
- typically jet black, can be dark green or brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-6 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from rapidly cooling extrusive felsic lava where crystals lack the time to grow. Most found today are relatively young geologically (less than 65 million years old) because it eventually devitrifies into stable minerals.
Uses & applications
Used historically for arrowheads and blades; modernly used for scalpel blades in heart surgery, ornamental jewelry, carvings, and as a protective gemstone in metaphysical communities.
Geological facts
Obsidian is so sharp that its edge can be thinner than a molecule of steel, potentially down to 3 nanometers. It is not technically a mineral because it lacks a crystalline structure.
Field identification & locations
Identify in the field by its smooth, glassy texture and distinctive 'c-shaped' conchoidal fractures. Common in volcanic regions such as Oregon (Glass Buttes), California, Iceland, and Mexico.
