igneous
Obsidian
Volcanic Glass (Amorphous silicon dioxide with trace impurities)
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically pitch black, dark brown, or green, Luster: Vitreous (glassy), Crystal Structure: Amorphous (non-crystalline), Cleavage: Conchoidal, Specific Gravity: 2.35 to 2.45
- Hardness
- 5 to 5
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically pitch black, dark brown, or green, Luster: Vitreous (glassy), Crystal Structure: Amorphous (non-crystalline), Cleavage: Conchoidal, Specific Gravity: 2.35 to 2.45
Formation & geological history
Formed from rapidly cooling felsic lava extruded from a volcano, preventing crystal growth and resulting in a natural glass. Geological age varies depending on the volcanic deposit.
Uses & applications
Used historically for sharp cutting tools, arrowheads, and blades, as well as in modern surgical scalpels, decorative items, and jewelry.
Geological facts
Because obsidian fractures with extremely sharp edges (conchoidal fracture), its edges can be just a few nanometers thick, making it sharper than high-quality steel surgical blades.
Field identification & locations
Easily identified by its glass-like appearance, conchoidal fracture, and dark color. Found worldwide in locations with recent volcanic activity, such as the western United States, Mexico, and Italy.
