igneous
Obsidian
Volcanic Glass (predominantly SiO2)
Hardness: 5-5.5 (Mohs), Color: Deep black, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (Conchoidal fracture), Specific Gravity: 2.35-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs), Color: Deep black, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (Conchoidal fracture), Specific Gravity: 2.35-2.6
Formation & geological history
Formed by the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. This specimen typically dates from the Cenozoic era, as volcanic glass decomposes over long geological time.
Uses & applications
Used in jewelry, decorative ornaments, surgical scalpel blades for high precision, and historically for tools/weapons like arrowheads and knives.
Geological facts
Obsidian breaks with a conchoidal fracture, creating edges that are thinner and sharper than the best steel scalpels. It was considered more valuable than gold by many ancient Mesoamerican cultures.
Field identification & locations
Identify by its jet-black color and smooth, curved (conchoidal) glass-like breakage patterns. Commonly found in volcanic regions such as the American Northwest (Oregon, California), Mexico, and Iceland.
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