Star Phenomena Resulting From Selective Absorption




Although selective absorption is the basic cause of the body color of any gem, on rare occasions the type and degree of absorption levels the specimen sensitive to the slight differences between incandescent light and daylight radiation. Artificial incandescent light contains a predominance of red wavelengths, whereas daylight contains a predominance of blue and green wavelengths. The result is a difference in the color of the stone, depending on the type of light to which it is exposed. This phenomenon is called CHANGE OF COLOR, and the most notable example of a stone displaying it is alexandrite, a variety of chrysoberyl.

At first it might seem that the difference between day lighting and artificial light would cause any stone to display a change of color when viewed under two sources alternately. However, in order for this to be obvious, the stone must be capable of absorbing a considerable portion of most of the wavelengths of light emitted by the light source, but a portion of the predominant wavelengths must be transmitted readily. Thus the alexandrite appears red in incandescent light, since it is transmitted with little absorption and the long wavelengths predominate in incandescent light. In daylight on the other hand, shorter wavelengths predominate, and since green is transmitted readily by the alexandrite while other short wavelengths are absorbed, the stone appears green. The more closely the stone approaches a pure green and a pure red, the better the quality.

A change of color occurs in other than natural chrysoberyl. Synthetic alexandrite is presently manufacture by Creative Crystals Inc. in California and by the Kyoto Ceramics Corporation in Japan. Both of these synthetic alexandrites have a distinct change of color. A change of color similar to that in natural alexandrite is also noted in the rather rare alexandrite like synthetic spinel. An alexandrite like synthetic corundum, which has often been incorrectly referred to as "synthetic alexandrite", changes from a gray bluish green in daylight, usually with purplish over stories, to a color similar to amethyst under artificial light. Occasionally, a natural sapphire, and more rarely a variety of beryl, spinel or tourmaline will show some change from one light source to another.