SELECTION OF CHATOYANT ROUGH
The term chatoyant is used to designate the silky sheen caused by very small reflecting inclusions of a fibrous nature which lie in parallel posi¬tion in certain gemstones. Strictly speaking, all star gems are chatoyant, but the chatoyancy occurs in several directions at once while in ordinary chatoyant gems, such as catseye chrysoberyl and catseye tourmaline, only one set of reflecting inclusions is present and therefore only one line of light crosses over the polished surface of the gem.
The principles for selecting suitable chatoyant rough are the same as outlined above for star stones. Sometimes the selection task is easier because some chatoyant gems possess this property so strongly that even the briefest inspection is enough to convince one that the effect really exists. Good examples are tigereye from Africa, some chrysoberyls and tourmalines, and rarely, some beryls. Figure 14 shows how a catseye gem is cut from chatoyant rough and indicates the proper shape of the rough if a good yield is expected.
In all star and catseye rough it is important to remember that the sharpest lines of light develop only if the finished cabochon is strongly curved over the top. If it is too shallow, the reflections tend to widen and become diffuse. A good rule of thumb is to select only those pieces of rough which are approximately ball-like in form. The finished cabo¬chon should be approximately hemispherical, or broadly oval, with the thickness of the gem from i/3 to 1/2 the breadth. If thicker gems are cut, they become grotesque, while thinner gems exhibit poorer optical effects. Further, the finest stars and eyes can only result if the inclusions are very slender, highly reflective, perfectly straight, sufficiently numerous to provide strong reflections, and evenly distributed throughout the crystal. It is very rare that all of these requirements are met in one gem, and naturally such a specimen commands a very high price.
Top-quality ruby, sapphire, and chrysoberyl rough displaying prom¬ising chatoyancy seldom leaves the country of origin without being cut locally and sold in this form. About the best that the amateur cutter can expect to obtain, or the professional for that matter, if he does not have direct trade connections into the countries producing such valuable rough, are smaller pieces of no more than several grams weight, or larger pieces which native cutters decided were too risky to cut because of un¬certainties about the outcome. Lately, much cheap, nearly opaque red corundum, euphemistically advertised as “ruby,” has come from India; it is a very poor approach to the real ruby in respect to quality, although it can be had in large crystals and in large quantities.