igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide)
Hardness of 5 to 5.5 on the Mohs scale, typically black to dark brown or green in color, vitreous (glassy) luster, concoidal fracture, completely amorphous crystal structure with no true crystal lattice.
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Physical properties
Hardness of 5 to 5.5 on the Mohs scale, typically black to dark brown or green in color, vitreous (glassy) luster, concoidal fracture, completely amorphous crystal structure with no true crystal lattice.
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 surgical scalpel blades and ornamental jewelry.
Geological facts
Obsidian can have microscopic magnetite crystals trapped inside that produce a sheen effect when oriented correctly. It was widely traded in the ancient world.
Field identification & locations
Easily identified by its glass-like appearance, smooth curved conchoidal fracture surfaces, and sharp edges. Found worldwide in areas with past volcanic activity.
