These are the most important of all, since on them largely depends the beauty of the various gem stones. In addition, the gemologist makes use of these characteristics to determine the nature of a given specimen, and with some practice and the use of certain instruments, a stone may be quickly and accurately defined The determination of the optical properties has the advantage of freedom from any risk of damage, and in most instances the specimen need not be removed from its setting if it is in a piece of mounted jewelry. The lapidary also, who cuts the rough stone, must be conversant with its optical properties, otherwise he will not get the best effect from the rough material.
For the purposes of discrimination, the two most useful instruments are the refractometer and the dichroscope. In most instances, the former gives the practised hand a definite reading, and from this reading, known as the refractive index (R.I.), the specimen may be distinguished. The dichroscope shows certain light effects in some instances, and their presence or absence will limit the nature of the stone under examination to a few species. This latter instrument costs very little; the refractometer, no matter which type is used, will cost more. When the necessary knowledge to use these two instruments has been acquired, they are the best (wo investments for those who have to decide quickly what a given specimen might be. To the jeweler, they are invaluable. But the use of these instruments involves some knowledge of light and particularly how rays of light are affected when they fall On such bodies as gem stones. It will be seen that crystal forma-I ion has a direct bearing on this large subject, of which we will touch only the fringe.
Here, we will assume that light consists of rays and travels in a straight line in order to explain various optical phenomena. These assumptions are not strictly correct. In describing the ac-(ion of light, we generally speak of rays, which may be regarded as portions of light enclosed by a hollow cone of very small angle, diverging from or converging to a point called the focus. The space or material through which light passes is termed a medium, and light travels in a straight line through the same homogeneous medium.
When rays of light, traveling in one medium, e.g. air, come in contact with the surface of another medium, e.g. a gem stone, they air, in general, broken up in three different ways. Some are reflected from the surface of the stone, others enter the stone and are refracted within the stone before emerging again, and the remainder is scattered or diffused. Reflected and refracted rays
follow definite paths, and their behavior has resulted in the establishment of certain “laws” which are of the utmost importance to the gemologist.
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