igneous
Obsidian
Volcanic glass (Amorphous silicon dioxide)
Hardness of 5 to 5.5 on the Mohs scale, typically jet black or dark brown, vitreous (glassy) luster, amorphous crystal structure (lacks a true crystal lattice), conchoidal fracture, specific gravity around 2.35 - 2.50.
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Physical properties
Hardness of 5 to 5.5 on the Mohs scale, typically jet black or dark brown, vitreous (glassy) luster, amorphous crystal structure (lacks a true crystal lattice), conchoidal fracture, specific gravity around 2.35 - 2.50.
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
Historically used for making arrowheads, cutting tools, and blades due to its extremely sharp edges; currently used in jewelry, ornamental objects, and experimentally in surgical scalpels.
Geological facts
Obsidian fractures with extremely sharp edges, producing cutting edges only a few nanometers thick. Despite being composed mostly of silica, it is classified as a mineraloid rather than a true mineral because it is a glass and lacks a crystalline structure.
Field identification & locations
Easily identified by its shiny, smooth, glass-like appearance and sharp conchoidal fracture lines. Commonly found near areas of past volcanic activity associated with rhyolitic lavas, such as the western United States, Mexico, and Italy.
