igneous
Obsidian
Obsidian (Volcanic Glass, ~70–75% SiO2)
Hardness: 5–6; Color: Typically black, can be grey, green, or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6
- Hardness
- 5–6
- Color
- Typically black, can be grey, green, or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–6; Color: Typically black, can be grey, 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 extremely rapidly with minimum crystal growth. Typically occurs in margins of rhyolitic lava flows or volcanic domes.
Uses & applications
Used in gemstone jewelry, ornamental carvings, and high-quality surgical scalpel blades. Historically used for making sharp tools like arrowheads and knives.
Geological facts
Because it doesn't have a crystal structure, obsidian edges can reach molecular thinness, making it sharper than high-quality steel scalpels. It is metastable and will eventually turn into quartz over millions of years.
Field identification & locations
Identify by its characteristic shell-like (conchoidal) fracture and glassy shine. Commonly found in volcanic regions like the American West (e.g., Yellowstone, Glass Butte) and Iceland.
