igneous
Obsidian
Obsidian (Amorphous Volcanic Glass, SiO2)
Hardness: 5-5.5 (Mohs scale); Color: Black, dark brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black, dark brown, or grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, dark brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich (rhyolitic) lava, preventing crystal growth. Usually found in volcanic regions from the Cenozoic era or younger, as glass devitrifies over millions of years.
Uses & applications
Historically used for arrowheads and knives; currently used as a gemstone in jewelry, as surgical scalpel blades for cleaner cuts, and as a decorative stone.
Geological facts
Obsidian is actually a liquid that has cooled so quickly it became a solid without crystallizing. Some varieties like Snowflake Obsidian contain white radially clustered crystals of cristobalite.
Field identification & locations
Identify by its glass-like texture and sharp, curved (conchoidal) edges. Found in the Western US (Oregon, California), Iceland, Japan, and Mexico. Use caution when handling as edges can be sharper than steel.
