igneous
Obsidian
Volcanic Glass (predominantly SiO2)
Hardness: 5-5.5 | Color: Deep black to dark brown | Luster: Vitreous/Glassy | Structure: Amorphous (non-crystalline) | Cleavage: None (conchoidal fracture) | Specific Gravity: 2.3-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 | Color: Deep black to dark brown | Luster: Vitreous/Glassy | Structure: Amorphous (non-crystalline) | Cleavage: None (conchoidal fracture) | Specific Gravity: 2.3-2.6
Formation & geological history
Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Found in geologically recent volcanic areas like the Western USA (Cenozoic era).
Uses & applications
Used in surgical scalpel blades for extreme sharpness, gemstone jewelry (cabochons), interior decoration, and historically for arrowheads and scrapers.
Geological facts
Obsidian breaks with a conchoidal fracture, creating edges that are thinner and sharper than a steel scalpel. Ancient Mayans and Aztecs used it for weapons and ritual mirrors.
Field identification & locations
Identify in the field by its distinctive glassy appearance and sharp, curved (conchoidal) breaking patterns. Commonly found in Oregon, California, and Iceland.
