igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35-2.6
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in geologically recent volcanic areas.
Uses & applications
Historically used for edged tools and weapons (arrowheads/knives). Modern uses include ornamental jewelry, scalpel blades for surgery, and collectibles.
Geological facts
Because it is glass, it is metastable and eventually devitrifies into quartz; therefore, obsidian older than the Cretaceous period is rarely found.
Field identification & locations
Identify by its extremely smooth, glass-like surface and sharp, curved (conchoidal) edges when broken. Commonly found in Oregon, Mexico, and Iceland.
