igneous
Obsidian
Obsidian (Volcanic Glass) SiO2 (70% or more)
Hardness: 5-5.5 (Mohs scale), Color: Jet black, Luster: Vitreous/Glassy, Structure: Amorphous (no crystal structure), Fracture: Conchoidal, Specific Gravity: 2.4
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black, Luster: Vitreous/Glassy, Structure: Amorphous (no crystal structure), Fracture: Conchoidal, Specific Gravity: 2.4
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in locations with recent volcanic activity like the Cascade Range and Yellowstone.
Uses & applications
Historically used for arrowheads and scrapers. Modern uses include surgical scalpel blades, jewelry, and ornamental carvings.
Geological facts
Unlike most rocks, obsidian is actually a natural glass and not a true mineral because it lacks a crystalline structure. It was highly prized by Mesoamerican cultures.
Field identification & locations
Identify by its deep black color, sharp edges, and signature conchoidal (curved/shell-like) fracture patterns. Common in volcanic regions of the US West, Mexico, and Iceland.
