CABOCHON MATERIAL
Flawlessness in cabochon material is not as crucial as it is in facet material; however, large flaws or surface cracks which tend to trap polishing powder, pores, soft inclusions which may pit during polishing, and other gross defects can result in unsightly gems. In very valuable materials such as star ruby and corundum, which depend for their value upon the inclusions creating the stars, great care is used in selecting the rough, because the brightness of the stars depends upon the basic clarity of the gemstone. If dark-colored inclusions are present, or small cracks and other defects, the reflections will be interfered with and the gem will be less beautiful. Some specific recommendations on star rough follow.
SELECTION OF ASTERIATED, OR STAR, ROUGH
It is not an easy task to judge quality in star sapphire because most crystals, supplied by the gravel deposits of Ceylon and Burma, are heavily abraded and not properly shaped to show the star even if the crystals are immersed in a suitable fluid. The inclusions which cause the star, or asterism, lie in only one series of parallel planes, as shown in Figure 13, with the optic axis at right angles to them. The first task is to determine the direction of this axis and to grind and polish away some material from one end of the crystal to impart the necessary spherical shape which allows the star to be seen. While some crystals are found which show a decided silky sheen at opposite ends, which mark the ends of the crystals and therefore the places where shaping should begin, this is not always the case, and some needless loss could occur unless the lapidary were sure. Fortunately, many crystals show some traces of crystal faces as shown in Figure 13. These faces are usually striated parallel to the planes containing the inclusions. Using them as clues, coupled with the appearance of sheen on opposite ends of the rough crystal, it is possible to safely shape and polish one end to check for asterism. The material removed in this preliminary shaping and polish¬ing operation needs only to be large enough to do the trick, sometimes a polished area of i/g” across being sufficient.
Star rubies may be more difficult to orient because some of the crystals are less regular in shape than sapphire crystals. However, one excellent clue, if the crystals are not too badly worn, is to look upon the surfaces for triangular markings and ridges which occur only on the ends of the crystals.
In both ruby and sapphire, frequent wetting with methylene iodide helps to detect the silky shimmer of the star-forming inclusions. All checking for asterism should be under a pinpoint source of light and not under a fluorescent light or an overhead fixture containing several light bulbs. If the sun is available, it provides the best light of all.