Precious opal

Composite Gemstones. Many types of composite or assembled gemstones have been made (1). Some are shown in Figure 3. In the United States a doublet has two pieces combined using a colodess cement. If three pieces are used, or if two pieces are assembled using a colored cement, the gemstone is a triplet. The use of composite stones has declined rapidly with the rise of inexpensive synthetics. Frequendy seen are opal doublets, where precious opal is backed by a black material. In opal triplets a thin sHce of precious opal is cemented between a black backing and a clear cover, usually of quartz. Additionally, insects and even fish have been inserted into amber. 

Precious opal is usually defined by the background color, black, white, or fire (orange). There are terms used to describe the distribution of the internal colors, and which colors are dominant. 

Sanders, J.V., Color of precious opal. Nature (London), 204, 1151, 1964. 

Figure 6.31 The structure of precious opal consists of ordered arrays of silica spheres with spacing of the order of the wavelength of light. These arrays diffract light and give rise to the colours seen in the gemstones...

Two-dimensional photonic band gap crystals can be created by building a two-dimensional array of voids or atoms in a transparent medium. Precious opal is an example of the three-dimensional photonic band gap crystal. 

Figure 14.27 The diffraction of light by ordered arrays of silica spheres gives colour to precious opals. The geometry of the diffraction is identical to that of the diffraction of X-rays...

They were named zeolite ( boiling stone ) in 1756 by Cronstedt, a Swedish mineralogist, who observed their emission of water vapor when heated. At the other size limit, opals constitute another example of a naturally occurring nanostmctured material. These gems are made up mainly of spheres of amorphous silica with sizes ranging from 150 nm to 300 nm In precious opals, these spheres are of approximately equal size and can thus be arranged in a three-dimensional periodic lattice. The optical interferences produced by this periodic index modulation are the origin of the characteristic iridescent colors (opalescence). 

J.V. Sanders Colour of precious opal. Nature 20it,... 

Aggregation Into Ordered Structures—Precious Opal... 

Structures of this type formed from very uniform particles of colloidal silica 100-500 nm in diameter are found in nature as precious opal. Because of the very curious nature of this phenomenon it is reviewed in some detail, including the analogous phenomenon involving uniform particles of other substances. 

The structure of precious opal was first described by Jones. Sanders, and Segnit (347) and Sanders (348), who demonstrated that the structure consisted of spheres of amorphous silica 150-350 nm in diameter showing an X-ray pattern devoid of any... 

The diffraction of light by precious opal gave diffraction patterns which were interpreted by analogy with X-ray diffraction theory. Sanders (356) found that the spherical silica particles were arranged hexagonally in layers which were usually stacked randomly. There are some parallel domains of ordered packing, usually face-centered cubic. 

General information on precious opal is available in several monographs (362-364). 

The colors of precious opal has already been discussed in connection with ordered aggregation and the interference colors of uniform layers of particles described under adsorption of particles on surfaces. 

A. Kalokerinos, Australian Precious Opal, Arco Publ. Co., New York, 1972. 

S. Tsunekawa, Yu. A. Bamakov, V.V. Poborchii, S.M. Samoilovich, A. Kasuya, and Y. Nishina Characterization of precious opals AFM and SEM observations, photonic band gap, and incorporation ofCdS nano-particles, Microporous Mater., 8(5-6) (1997) 215—2 2... 

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