An iridescent, rainbow-like sheen ( rainbow obsidian) is caused by inclusions of magnetite nanoparticles. These bubbles can produce interesting effects such as a golden sheen ( sheen obsidian). Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern ( snowflake obsidian). Iron and magnesium typically give the obsidian a dark brown to black color. Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Having a low water content when newly formed, typically less than 1% water by weight, obsidian becomes progressively hydrated when exposed to groundwater, forming perlite. This breakdown of obsidian is accelerated by the presence of water. Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. Crystalline rocks with obsidian's composition include granite and rhyolite. Obsidian consists mainly of SiO 2 ( silicon dioxide), usually 70% or more. Though obsidian is usually dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. It is sometimes classified as a mineraloid. Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline in addition, its composition is too complex to comprise a single mineral. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis. Obsidian is the rock formed as a result of quickly cooled lava, which is the parent material. The translation into English of Natural History written by the elder Pliny of Rome shows a few sentences on the subject of a volcanic glass called Obsian, so named from its resemblance to a stone ( obsiānus lapis) found in Ethiopia by Obsius, a Roman explorer. among the various forms of glass we may reckon Obsian glass, a substance very similar to the stone found by Obsius in Ethiopia. Obsidian talus at Obsidian Dome, California Obsidian is hard and brittle it therefore fractures with very sharp edges, which were used in the past in cutting and piercing tools, and it has been used experimentally as surgical scalpel blades. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. For other uses, see Obsidian (disambiguation).
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