igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5–5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous/Glassy, Crystal Structure: Amorphous, Cleavage: None (Conchoidal fracture), Specific Gravity: 2.35–2.6
- Hardness
- 5–5
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Physical properties
Hardness: 5–5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous/Glassy, Crystal Structure: Amorphous, 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 crystal growth. Usually relatively young geologically (Cretaceous to Holocene) because it eventually devitrifies into crystalline rock.
Uses & applications
Used for surgical scalpel blades, ornamental jewelry, decorative carvings, and historically for sharp tools like arrowheads and knives.
Geological facts
Obsidian is not a mineral because it lacks a crystalline structure; it is considered a mineraloid. Known as 'Dragon Glass' in popular culture, it has been traded by humans for thousands of years.
Field identification & locations
Identify by its extremely smooth, glass-like surface and sharp conchoidal (shell-like) fractures. Common in volcanic regions such as the American Northwest, Iceland, Mexico, and Japan.
