igneous
Obsidian
Amorphous volcanic glass (primarily SiO2)
Hardness: 5-5.5 (Mohs), Color: Black/Dark green, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture)
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs), Color: Black/Dark green, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture)
Formation & geological history
Formed by the rapid cooling of silica-rich felsic lava, preventing crystal growth. Most specimens are relatively young geologically (Cenozoic) because glass is metastable and devitrifies over time.
Uses & applications
Used in ornamental jewelry, decorative items, and historically for ultra-sharp surgical scalpels and prehistoric tools like arrowheads and blades.
Geological facts
Small rounded nodules of obsidian are often called 'Apache Tears.' They exhibit a unique translucency when held up to a strong light source.
Field identification & locations
Identified by its deep black color, high gloss, and curved, shell-like (conchoidal) fracture lines. Commonly found in volcanic regions like the American West, Mexico, and Iceland.
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