igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-6 (Mohs); Color: Black, deep brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-6 (Mohs)
- Color
- Black, deep brown, or grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Black, deep brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is relatively young geologically (mostly Cenozoic) because it is unstable and eventually devitrifies into crystals.
Uses & applications
Used historically for tools/weapons (arrowheads, blades); modern uses include surgical scalpels, ornamental jewelry, cabochons, and decorative architecture.
Geological facts
Because obsidian lacks crystal structure, its fracture edges can reach almost molecular thinness, making it much sharper than high-quality steel scalpels. It remains a popular material for 'knapping' by modern hobbyists recreating ancient tools.
Field identification & locations
Identify in the field by its characteristic smooth, curved (conchoidal) fracture surfaces that look like broken glass. Commonly found in volcanic regions such as the American Northwest, Mexico, Iceland, and Japan.
