igneous
Obsidian
Volcanic Glass (predominantly SiO2)
Hardness: 5-5.5 (Mohs); Color: Black to dark brown/green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black to dark brown/green
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Black to dark brown/green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.4
Formation & geological history
Formed when extrusive felsic lava cools rapidly with minimal crystal growth. Typically found in volcanic areas from the Cenozoic era onwards (as it devitrifies over time).
Uses & applications
Historically used for arrowheads and blades; currently used in scalpel manufacturing, jewelry (cabs/beads), and as an ornamental stone.
Geological facts
Because it is glass and not a crystal, it breaks into edges that are thinner and sharper than a high-quality steel scalpel. Ancient Aztecs used it for sacrificial knives and mirrors.
Field identification & locations
Identify by its characteristic conchoidal (curved, shell-like) fracture and smooth, glassy texture. Found globally in volcanic regions like the American West (Glass Buttes), Iceland, and Mexico.
