igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 | Color: Typically black or dark grey | Luster: Vitreous/Glassy | Structure: Amorphous (non-crystalline) | Fracture: Conchoidal (shell-like) | Specific Gravity: 2.35-2.60
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 | Color: Typically black or dark grey | Luster: Vitreous/Glassy | Structure: Amorphous (non-crystalline) | Fracture: Conchoidal (shell-like) | Specific Gravity: 2.35-2.60
Formation & geological history
Formed by the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. It is relatively young geologically, as it eventually devitrifies into crystals over millions of years.
Uses & applications
Historically used for making sharp tools like arrowheads and knives. Today used as a gemstone in jewelry, in high-precision scalpel blades for surgery, and as an ornamental stone.
Geological facts
Obsidian can be so sharp that its edge can reach molecular thinness, making it sharper than high-quality steel scalpels. It is technically a 'mineraloid' because it lacks a crystal structure.
Field identification & locations
Identified by its distinctive glassy appearance and razor-sharp, curved conchoidal fracture edges. Found in volcanic regions like the American West, Iceland, and Mexico. Collectors look for unique variations like 'snowflake' or 'mahogany' patterns.
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