igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-6 (Mohs); 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
- 5-6 (Mohs)
- Color
- Black, sometimes dark green or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); 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 from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Common in areas with recent volcanic activity (Cenozoic era).
Uses & applications
Historically used for arrowheads and scrapers; currently used for surgical scalpel blades, jewelry, and ornamental carvings.
Geological facts
Because it is a glass and not a crystal, obsidian is technically metastable and will eventually hydrate and turn into perlite over millions of years. It was highly prized in pre-Columbian Mesoamerica for tools and ritual objects.
Field identification & locations
Identify by its extremely smooth, glass-like texture and sharp, curved conchoidal fractures. Common in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.
