igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark green, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Fracture: Conchoidal (shell-like curves), Specific Gravity: 2.4
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark green, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Fracture: Conchoidal (shell-like curves), Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich lava (rhyolitic) where cooling happens so quickly that crystals do not have time to grow. Found in geologically young volcanic areas.
Uses & applications
Historically used for surgical scalpels and arrowheads due to its sharp edges; used today in jewelry (cabochons), massage stones, and ornamental carvings.
Geological facts
Obsidian can produce a cutting edge many times sharper than high-quality steel scalpels. Unlike most rocks, it is technically a natural glass rather than a mineral because it lacks a crystal structure.
Field identification & locations
Identify by its smooth glassy texture, sharp edges when broken, and characteristic curved (conchoidal) surface. Commonly found in Oregon, Mexico, and Iceland.
