The word "chatoyancy" comes from a contraction of the French words chat for cat and oeil for eye; it aptly describes this odd reflection phenomenon. The cat's-eye effect is a sharp single band of light running like a brilliant slit across an oval-shaped stone. It looks for all the world like a glowing cat's eye. The light band is multiple reflection from thousands of needlelike inclusions in the gemstone, all running parallel to each other. Interestingly, the thinner and more numerous the inclusions, the sharper and brighter the eye. These needlelike structures may be the mineral species rutile, or may even be extremely thin, empty, capillary tubes. Although the finest cat's-eyes occur in chrysoberyl, they have also been found in some tourmaline, ruby, sapphire, garnet, spinel, and even quartz.
If, by chance, the needle-like inclusions are lined up in two or even three directions related to the crystal structure, the chatoyancy becomes more complex and two or three light bands are reflected. Thus the very popular rubies and sapphires showing asterism, or a star effect, are merely displaying their chatoyancy in three directions with the bands of light intersecting at a single point. Of course, it is necessary to cut such a stone carefully in the proper crystal direction, so that the intersection of the light bands falls at the center of the peak of the rounded gem. Unfortunately, if the inclusions are lacking or more sparse in one crystal direction than in another, a star with weak or missing legs results. All star and cat's-eye stones perform better if viewed under a single point source of light, such as the sun or an incandescent light bulb. Other light sources are likely to be so diffuse as to produce only diffuse reflections.