igneous
Obsidian
Volcanic Glass (70% or more SiO2)
Hardness: 5-6 (Mohs); Color: Jet black to dark grey/green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4
- Hardness
- 5-6 (Mohs)
- Color
- Jet black to dark grey/green
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Jet black to dark grey/green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4
Formation & geological history
Formed through the extremely rapid cooling of felsic (silica-rich) lava, preventing crystal growth. Usually found in volcanic regions of any geological age where high-silica eruptions occurred.
Uses & applications
Historically used for cutting tools, arrowheads, and mirrors. Modern uses include surgical scalpel blades, jewelry (cabochons), and ornamental home decor.
Geological facts
Because it is glass, obsidian is chemically unstable over long geological time and eventually 'devitrifies' into quartz crystals. It has one of the sharpest edges known to man, far finer than a steel blade.
Field identification & locations
Identify by its distinct conchoidal fracture and sharp, glass-like edges. Commonly found in Oregon, Mexico, Iceland, and Italy. Collectors look for unique patterns like 'Snowflake' or 'Rainbow' sheen.
