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Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence). This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids. Different opalescent colors are produced by colloids of differing sizes.
Precious 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 150–300 nm (5.9 × 10 −6 –1.18 × 10 −5 in) in diameter in a hexagonal or cubic close-packed lattice.
Each of the three notable types of opal – precious, common, and fire [3] – display different optical effects; therefore, the intended meaning varies depending on context. The general definition of opalescence is a milky iridescence displayed by an opal, which describes the visual effect of precious opal very well, and opalescence is ...
The Yowah opal field in the Shire of Paroo. The Yowah nut is a type of precious opal, found within the Yowah opal fields situated in Yowah, Shire of Paroo, South West Queensland, Australia since the latter part of the 19th century. [1] These opals are known for their distinctive nut-like shape, opalescent patterns, and vibrant colours.
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The name moonstone derives from the stone's characteristic visual effect, called adularescence (or schiller), which produces a milky, bluish interior light. This effect is caused by light diffraction through alternating layers of orthoclase and albite within the stone.
The micrometre-scale spheres that make up precious opal exhibit some X-ray diffraction patterns that are similar to that of cristobalite, but lack any long-range order so they are not considered true cristobalite. In addition, the presence of structural water in opal makes it doubtful that opal consists of cristobalite. [12] [13]
Unpolished sunstone. The optical effect is due to reflections from inclusions of red copper, hematite, or goethite, in the form of minute scales, which are hexagonal, rhombic, or irregular in shape, and are disposed parallel to the principal cleavage-plane.