Light Crystal Opal Classified by the Opal industry as Natural Opal Type 1 - Crystal Opal is defined when the Opal is presented in one piece in its natural state apart from cutting or polishing, and is of substantially homogenous chemical composition and is translucent (free of oxides). Genuine Natural Australian Crystal Opal occurs in limited quantity in all of the Australian Opal fields (Andamooka, Lightning Ridge in NSW, Coober Pedy in South Australia and the Queensland Boulder Opal Fields). A hydrated amorphous form of silica SiO2nH2O consisting of spheres of silicon dioxide molecules arranged in regular, closely packed planes. The Opal is pure hydrated silica SiO2nH2O free from oxides (Magnezium or Iron) so it has a clear crystal or seethrough translucency. Precious Crystal Opal shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. At microscopic scales, precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice.