igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5–5.5 (Mohs scale); Color: Typically black, but can be brownish or green; Luster: Vitreous (glassy); Crystal Structure: Amorphous (structureless); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5–5
- Color
- Typically black, but can be brownish or green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 (Mohs scale); Color: Typically black, but can be brownish or green; Luster: Vitreous (glassy); Crystal Structure: Amorphous (structureless); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich rhyolitic lava, preventing crystalline growth. It is found in geologically recent volcanic areas, typically from the Tertiary to Quaternary periods.
Uses & applications
Historically used for sharp cutting tools (scalpels, arrowheads). Modern uses include jewelry (cabochons), ornamental carvings, and high-quality surgical blades.
Geological facts
Because it is glass and lacks a crystal structure, obsidian is metastable and eventually 'devitrifies' or crystallizes into quartz and feldspar over millions of years. This is why very old obsidian (Precambrian) is rare.
Field identification & locations
Identified by its extremely smooth, curved (conchoidal) fracture surfaces and sharp edges. Common in volcanic regions such as the American West, Iceland, and Italy. Practitioners of 'flint-knapping' value it highly.
