Sunrise Opals, Quilpie, QLD

Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example. In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification.The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color.The veins of opal displaying the best play of color are often quite thin, but to the naked eye appear to have an amazing depth due to a layer of black or dark potch backing the prescious opal in a lightening Ridge stone or ironstone and manganese backing the prescious opal in boulder opals found in western Queensland.