igneous
Obsidian
Obsidian (Volcanic Glass, SiO2)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark brown with translucent edges, Luster: Vitreous/Glassy, Crystal 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 scale), Color: Jet black to dark brown with translucent edges, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None/Conchoidal fracture, Specific Gravity: 2.35-2.6
Formation & geological history
Obsidian is an extrusive igneous rock formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in volcanic regions of any geological age, though most is relatively young (Quaternary).
Uses & applications
Used in jewelry, surgical blades, decorative art, and gemstone collecting. Historically used for arrowheads and cutting tools.
Geological facts
Because it is a natural glass, obsidian breaks with extremely sharp edges. Some modern surgeons use obsidian scalpels because they provide a cleaner cut with less scarring than steel.
Field identification & locations
Identify by its glass-like texture and conchoidal (shell-like) fracture patterns. Found in volcanic areas such as Oregon, Iceland, and Mexico. Collectors should look for unique internal sheens.
