igneous
Obsidian
Obsidian (Volcanic Glass, SiO2)
Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black, sometimes dark green or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, which prevents the growth of crystals. Predominantly found in volcanic areas from the Cenozoic era due to its susceptibility to hydration over long periods.
Uses & applications
Used in the manufacture of surgical scalpel blades, jewelry, decorative ornaments, and historically for sharp tools and weapons like arrowheads.
Geological facts
Obsidian is technically a glass, not a mineral, because it lacks a crystalline structure. Ancient Aztecs used obsidian mirrors for divination and rituals.
Field identification & locations
Identify by its characteristic shiny, glassy appearance and shell-like (conchoidal) fractures with sharp edges. Commonly found in locations like the Western US, Mexico, and Iceland. Collectors look for unique varieties like Snowflake or Mahogany obsidian.
