Step, brilliant, mixed and calibre styles are used for transparent material, although very dark and/or semi-transparent stones are sometimes cut in cabochon. Pale stones are often cut with heavy backs to deepen the color. In order to produce the most attractive color in either transparent ruby or sapphire, the table of the stone should be oriented at right angles to the optic axis. When this is not done, rubies will appear orangey red, instead of lightly purplish red, and blue, sapphires will appear greenish blue, instead of slightly violetish blue, thus reducing their value and desirability. When orienting asteriated material, the girdle must be placed parallel to the plane in which the surface sheen is noted on the crystal (i.e. perpendicular to the optic axis); otherwise, the star will not be centered at the apex of the cabochon. Fortunately, a stone with a properly oriented star will automatically have the best color possible.
Untwined and most lightly flawed corundum is not heat sensitive to the various operations of the fashioning process, nor does cleavage pose a problem; it is, however, expensive to cut and polish. Because of its hardness, the techniques used for other materials are unsatisfactory. Cabochon material can be cut on silicon-carbide wheels, but the weels wear down at a tremendous rate. Large stones should be trimmed as closely as possible with the diamond saw. Sanding can be accomplished on a very coarse-grit cloth only. Cabochons are polished on grooved wooden wheels or pellen discs with 1200-grit diamond powder, followed by 6400 grit powder. Care must be exercised in this operation, however, since the heat generated may loosen the stone on the dop; therefore, occasional cooling is recommended.
Faceting is accomplished on diamond-impregnated cooper laps, or electroplated diamond surface laps, and polishing is done on tin or zinc laps with 14,000 to 50,000 diamond powder and oil. The finishing can sometimes be improved by following the diamond-powder polishing with tripoli. Suggested angles for the brilliant cut are 37°for the main crown facets and 42°for the main pavilion facets.
