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Tuesday, 18 September 2007

dispersion. The property of transpa­rent Gemstones to separate white light into the spectral colors. The interval between such colors varies in different gemstones, and is usually expressed by the measure of the dif­ference between the refractive indi­ces of the red ray and the violet ray. diamond has the highest dispersion (.044) of any of the popular, natural, colorless gems. See fraunhofer lines.

Di'Yag. Trade name for man-made yttrium aluminum garnet (YAC). Djevalite. Trade name for man-made cubic zirconia.

dodecahedral cleavage (doe'-dek-ah-he"-dral). Cleavage in six planes, parallel to pairs of opposite faces

of the dodecahedron. Although dia­mond may show such cleavage, it is not easily developed. See dodeca­hedron, CLEAVAGE, CLEAVAGE (IN DIAMOND).

dodecahedron (doe'-dek-ah-he"-dron). One of the seven basic forms in the highest symmetry (hexoctahe-dral) class of the cubic, or isometric, crystal system. It has 12 rhomb-shaped faces, each of which inter­sects two of the crystallographic axes and is parallel to the third. It is also called the rhombic-dodecahedron. See cubic system, hexoctahedral class.

 
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