igneous
Obsidian
Volcanic Glass
Hardness of 5 to 5.5 on the Mohs scale, typically black or dark brown in color, vitreous (glassy) luster, amorphous crystal structure with conchoidal fracture, specific gravity around 2.35 to 2.6.
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Physical properties
Hardness of 5 to 5.5 on the Mohs scale, typically black or dark brown in color, vitreous (glassy) luster, amorphous crystal structure with conchoidal fracture, specific gravity around 2.35 to 2.6.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Geological age varies widely depending on the volcanic deposit.
Uses & applications
Used historically for sharp cutting tools, arrowheads, and blades, as well as in modern decorative items, jewelry, and occasionally surgical scalpels.
Geological facts
Because it cools so quickly without forming a crystal lattice, obsidian is technically a mineraloid rather than a true mineral.
Field identification & locations
Easily identified by its dark color, glass-like appearance, and sharp conchoidal fracture edges. Commonly found in areas with past volcanic activity such as the western United States, Italy, and Mexico.
