igneous
Obsidian
Volcanic Glass (predominantly SiO2)
Hardness: 5–5.5 (Mohs scale); Color: Black (jet black); Luster: Vitreous/Glassy; Crystal Structure: Amorphous (lacks crystal structure); Cleavage: Conchoidal (shell-like) fracture; Specific Gravity: 2.4
- Hardness
- 5–5
- Color
- Black (jet black)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 (Mohs scale); Color: Black (jet black); Luster: Vitreous/Glassy; Crystal Structure: Amorphous (lacks crystal structure); Cleavage: Conchoidal (shell-like) fracture; Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, which prevents crystal growth. It is typically geologically young (Cenozoic) because it eventually devitrifies into crystalline rock over millions of years.
Uses & applications
Historically used for arrowheads and blades. Currently used in jewelry, decorative items, and high-precision surgical scalpel blades because it can be fractured to an edge much thinner than steel.
Geological facts
Because it is an amorphous solid (glass) rather than a crystalline structure, it is often called a 'mineraloid' rather than a true mineral. Large flows can be found at the Obsidian Cliff in Yellowstone National Park.
Field identification & locations
Identify by its characteristic conchoidal fracture (curved, glass-like breaks) and extremely sharp edges. Commonly found in volcanic regions such as the American Northwest, Mexico, and Iceland. Collectors look for unique varieties like snowflake or mahogany obsidian.
