Precious opal shows a variable interplay of colors as it has an internal structure. Under a microscope you can see precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the defraction of light passing through the microstructure of the opal. It is the regularity and size of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction.
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.
( prescious opal )
( potch opal )
Thin Seam of Prescious Opal ( Boulder Opal )
Prescious Opal Potch Backing ( Lightning Ridge )