igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurities)
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black to dark grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Usually found in volcanic regions of geologically recent age, often from the Pliocene to Holocene.
Uses & applications
Historical use for sharp tools and weapons (arrowheads/knives); modern use in jewelry, surgical scalpels, and decorative ornaments.
Geological facts
Because it is a glass and not a crystal, obsidian is chemically unstable over geological time and eventually devitrifies into ordinary minerals. It breaks with a conchoidal fracture, producing edges thinner than a diamond scalpel.
Field identification & locations
Identify by its glass-like appearance and curved, shell-like (conchoidal) fracture patterns. Frequently found in volcanic fields like Glass Buttes, Oregon or the Yellowstone area.
