igneous
Obsidian
Volcanic Glass (Amorphous silicon dioxide with impurities)
Hardness of 5 to 5.5 on the Mohs scale, color typically black but can be brown, tan, or translucent, vitreous (glassy) luster, amorphous crystal structure with conchoidal fracture, specific gravity of 2.35 - 2.60.
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Physical properties
Hardness of 5 to 5.5 on the Mohs scale, color typically black but can be brown, tan, or translucent, vitreous (glassy) luster, amorphous crystal structure with conchoidal fracture, specific gravity of 2.35 - 2.60.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth.
Uses & applications
Historically used for arrowheads, cutting tools, and blades; currently used in jewelry, ornamental objects, and experimentally for surgical scalpel blades.
Geological facts
Because it cools so quickly, no crystals can form, making it a natural mineraloid glass rather than a true mineral.
Field identification & locations
Easily identified by its extremely sharp conchoidal fracture edges and glossy, glass-like appearance. Commonly found worldwide near volcanic areas with rhyolitic flows.
