Igneous
Obsidian
Volcanic Glass (comprised primarily of SiO2)
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be grey or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- typically jet black but can be grey or green
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be grey or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Most specimens belong to the Cenozoic era, as volcanic glass eventually devitrifies into crystal over millions of years.
Uses & applications
Historically used for arrowheads and blades; currently used in high-end surgical scalpels, jewelry (cabochons), and ornamental carvings.
Geological facts
Because it has no crystal structure, obsidian can be fractured to an edge only a few molecules thick, making it significantly sharper than high-quality steel scalpels. It is technically a mineraloid, not a true mineral, because it lacks a crystal lattice.
Field identification & locations
Identify in the field by its distinct glassy texture, sharp edges, and unique radial 'conchoidal' fracture lines. Common in volcanic regions such as the American West (Oregon, California), Iceland, and Mexico.
