In general, the monosized silica spheres in an opal gem are arranged as a polycrystal in which various grains give colors of different wavelength, since diffraction conditions depend on orientation of the grain. This Brazilian opal, however, has a band ed structure of two different ordered phases: AB2 and AB13, where A is "large" spheres with a diameter of 0.36 microns and B is "small" spheres with a diameter of 0.21 microns. (The structure can also be described as a superlattice.) The optical effects i n this gem are described as being different from conventional opal in that it is very "flashy"; grains of color appear and disappear suddenly as the gem is turned. In addition, the gem is much more colorful than usual. In diffraction language, this means that the Bragg condition is frequently met, with a greater than usual number of reciprocal lattice points within the optical range of reciprocal space. This Brazilian gem has some grains with cerise colors-- a combination of concurrent diffraction of red and violet wavelengths made possible by the superlattice structure.
Source: J.V. Sanders, "Close-packed structures of spheres of two different sizes, I. Observations on natural opal," Phil. Mag. A, Vol. 42, No. 6, 705-720 (1980).
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