igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark brown, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.60
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark brown, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.60
Formation & geological history
Obsidian forms when silica-rich lava cools extremely rapidly, preventing the growth of mineral crystals. This typically occurs at the edges of basaltic or rhyolitic lava flows or when lava comes into contact with water. It can be found in volcanic terrains from the Cenozoic era.
Uses & applications
Historically used for sharp tools, arrowheads, and blades. Modernly used for surgical scalpels, decorative jewelry, and ornamental carvings.
Geological facts
Known as 'nature's glass,' its conchoidal fracture creates edges thinner and sharper than a steel scalpel. The Aztec civilization highly valued it for ritual knives and mirrors.
Field identification & locations
Identify by its smooth, glassy texture and distinctive shell-like (conchoidal) fractures. Found in volcanic areas like the Western USA, Iceland, and Mexico. Collectors look for unique varieties like Snowflake or Mahogany obsidian.
