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Brief Description: Known since the ancient times. It is referred to as opallios (Dioskourides) and opalus (Plinius). The name derives from the Sanskrit term upala = gemstone. Its chemical formula is SiO2?nH2O and it belongs to the category Tectosilicates (SiO2 group). Its lustre is vitreous, waxy, sebaceous to resinous. Common opal is usually found in shades of yellow, brown, red, green, grey and blue. The dark colouring is due to fine-grained inclusions. It is also found colourless or white (vitrite). In the noble opalescent variety, iridescence (opalescence) and nice colour interplay can be noticed. Its specific gravity ranges from 2.0 ? 2.2 and it is transparent to opaque. Its hardness is 5½ - 6½ and it is not fusible. It usually occurs in solid, uneven masses, in spheroid encrustations, in botryose and stalactiform aggregates and in pseudomorphs like fossils and fossilized wood. It is deposited in hot springs in small depths of meteoric water or hypogene solutions of low temperature. It surrounds or fills in rock cavities of all types and substitutes wood and shells. The largest opal concentrations can be found in the silicon shells of organisms that secrete silicon dioxide (sponges, radiolarian, diatoms). It often presents colour interplay known as opalescence. The noble varieties (vitrite, girasol) are used as semiprecious and precious stones. Diatomite is used as filter of sugar, juices and other drinks, in fertilizers, in colours, as insulation and anti-acoustic and as soft abrasive. There is an artificial opal as well, with the same physical and chemical properties as the natural one. 
Bibliography: Ευρετήριο ορυκτών ΑΠΘ στην ιστοσελίδα http://www.geo.auth.gr/106/az_gr.htm Τσιραμπίδης Α. Ο ορυκτός πλούτος της Ελλάδας. Εκδόσεις Γιαχούδη Θεσσαλονίκη, 2005, σελ. 391. Berry, L.G., Mason, B. and Dietrich, R.V. (1983). Mineralogy. W. H. Freeman and Company, San Francisco, 561 pp. Σαπουντζής, Η. και Χριστοφίδης, Γ. (1985). Ορυκτοδιαγνωστική. University Studio Press, Θεσσαλονίκη, 241 σ.