igneous
Obsidian
Volcanic Glass (Amorphous Silica)
Hardness: 5-5.5 (Mohs scale); Color: Black, dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black, dark brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from extrusive igneous activity when felsic lava (rich in silica) cools so rapidly that individual crystals do not have time to grow. It is common in volcanically active regions as young as the Quaternary period.
Uses & applications
Historically used for making extremely sharp arrowheads, knives, and surgical scalpels. In modern times, it is used for jewelry, decorative carvings, and as a semi-precious collector's stone.
Geological facts
Obsidian fractures with sharpness down to nearly molecular levels; it is actually sharper than the highest quality steel scalpels. Unlike most rocks, it is not a mineral because it lacks a regular crystal structure.
Field identification & locations
Identify by its characteristic glassy appearance and smooth, curved (conchoidal) fracture edges. Found in areas of past or present volcanic activity like the American West (Glass Buttes), Iceland, and Japan.
