igneous
Obsidian
Obsidian (Volcanic Glass, primarily SiO2)
Hardness: 5–5.5 (Mohs scale); Color: Black to dark grey; Luster: Vitreous/Glassy; Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.
- Hardness
- 5–5
- Color
- Black to dark grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 (Mohs scale); Color: Black to dark grey; Luster: Vitreous/Glassy; Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava (rich in silica) cools so rapidly that crystals do not have time to grow. Most specimens are geologically young, typically from the Cenozoic era, as glass eventually devitrifies into minerals over millions of years.
Uses & applications
Historically used for making sharp tools like arrowheads and knives. Modern uses include surgical scalpel blades, jewelry (cabochons), and ornamental carvings.
Geological facts
Because of its lack of crystal structure, obsidian breaks into pieces with edges thinner and sharper than high-quality steel scalpels. It was highly prized in Mesoamerican cultures for both weaponry and ritual mirrors.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and sharp, translucent edges. It is commonly found in volcanic regions such as the Western United States, Mexico, Iceland, and Italy. Collectors should look for smoothness and the absence of visible grains.
