igneous
Obsidian
Volcanic Glass
Hardness of 5 to 5.5 on the Mohs scale, typically black or dark brown to translucent, vitreous (glassy) luster, amorphous crystal structure (lacks a true crystalline lattice), conchoidal fracture, specific gravity around 2.35 to 2.45.
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Physical properties
Hardness of 5 to 5.5 on the Mohs scale, typically black or dark brown to translucent, vitreous (glassy) luster, amorphous crystal structure (lacks a true crystalline lattice), conchoidal fracture, specific gravity around 2.35 to 2.45.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in areas of recent volcanic activity.
Uses & applications
Historically used for making cutting tools, arrowheads, and blades due to its extremely sharp edges; currently used in jewelry, ornamental objects, and experimentally in surgical scalpel blades.
Geological facts
Because obsidian is a glass and not a true mineral, it does not have a crystalline structure. When broken, it exhibits some of the sharpest known edges of any material, often only a few molecules thick.
Field identification & locations
Easily identified in the field by its intense vitreous luster, dark color, and sharp conchoidal fracture lines resembling broken glass. Commonly found worldwide near volcanic regions such as the western United States, Mexico, and Italy.
