igneous
Obsidian
Volcanic Glass
Hardness of 5 to 5.5 on the Mohs scale, typically black to dark brown or green, vitreous (glassy) luster, amorphous crystal structure with 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 to dark brown or green, vitreous (glassy) luster, amorphous crystal structure with 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.
Uses & applications
Used historically for sharp cutting tools, arrowheads, and blades, as well as in modern decorative items, jewelry, and experimental surgical scalpels.
Geological facts
Because it cools so rapidly, no crystals can form, making it a natural glass rather than a true mineral. It breaks with extremely sharp edges.
Field identification & locations
Easily identified by its glass-like appearance, dark color, and characteristic conchoidal fracture. Found in regions with recent volcanic activity such as the western United States, Italy, Mexico, and Japan.
