In gemology, opalescence is applied to the hazy-milky- turbid sheen of common or potch opal which does not show a play of color. The term opalescence is commonly used to describe this unique and beautiful phenomenon, which in gemology is termed play of color. In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during its formation. The notion that opals are photonic crystals for visible light was expressed in 1995 by Vasily Astratov's group.
This is the basis of the optical band gap in a photonic crystal. Visible light cannot pass through large thicknesses of the opal. The process can be described by Bragg's law of diffraction. The colors that are observed are determined by the spacing between the planes and the orientation of planes with respect to the incident light. Where the distance between the regularly packed planes of spheres is around 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 regularity of the sizes and the packing of these spheres is a prime determinant of the quality of precious opal.
Sanders in the mid-1960s that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal. At microscopic scales, precious opal is composed of silica spheres some 150–300 nm (5.9 ×10 −6–1.18 ×10 −5 in) in diameter in a hexagonal or cubic close-packed lattice. Precious opal shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. Precious opal replacing ichthyosaur backbone, as a display specimen in South Australian Museum This multicolored rough opal from Coober Pedy, South Australia, displays nearly every color of the visible spectrum. Precious opal Precious opal consists of spheres of silica molecules arranged in regular, closely packed planes (idealized diagram).
Black opal is considered the rarest, while white, gray, and green opals are the most common. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque, and the background color may be white, black, or nearly any color of the visual spectrum. Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light." The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Precious opal displays play-of-color ( iridescence) common opal does not. There are two broad classes of opal: precious and common. The name opal is believed to be derived from the Sanskrit word upala ( उपल), which means 'jewel', and later the Greek derivative opállios ( ὀπάλλιος), which means 'to see a change in color'. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals.
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