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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.

With the development of analytical chemistry, the blowpipe method has given way to the “wet” method once more, but this is not very suitable or useful to the gemologist as it is often restricted to minerals which are soluble in some liquid. A recent method is the noting of calorimetric reactions in the presence of a variety of reagents. By these means, a drop of the reagent selected is allowed to come into contact with a drop of the solution ‘being tested, and the resulting color is sufficient to indicate which elements are present. The method in use and the reagents required are described in a paper read by J. H. Watson on November 1st 1934, before the Mineralogical Society of London. The details do not come within the scope of this work, nor would the results be of much practical use to the gem dealer or collector.

Actually, an analysis of the earth’s crust, based on a series of averages taken, shows that oxygen and silicon make up the major portion. These elements are, of course, found in chemical combination with others, but nearly half the weight is oxygen, while more than a quarter is silicon. Silicon, iron, aluminum, and magnesium exist in large quantities, the proportions being about 46 per cent oxygen, 27 per cent silicon, 8 per cent aluminum, 5 per cent iron, 4 per cent calcium, and 3 per cent potassium, other elements making up the remainder. Almost all the minerals are found in combination with others, the rough material being known as ores, and from these the single mineral or metal must be extracted. Gem stones are not extracted, except by simple separation, from ores. They are found as we know them, with the exception of shaping and polishing which has been applied to improve their beauty.

At present, about 92 different stable elements are known, and all minerals are composed of one or more of these elements. For conciseness, these elements are generally denoted by a symbol, e.g. C for carbon, Al for aluminum, Mg for magnesium, and the chemist has a simple means of showing not only the constituent elements of a substance by such symbols but also their compara tive weights.
Elements combine in definite proportions only, and each proportion is a definite character of that element, which is called its atomic weight. For instance, the atomic weight of hydrogen is 1, of carbon 12, of oxygen 16, and of magnesium 24. Certain gases and substances are capable of forming acids in conjunction with hydrogen, or hydrogen and oxygen. These acids combine with greater or less avidity with other elements which do not form acids, and which are termed bases. Such a combination of an acid with a base produces a compound solution called a salt. All the salts whose names terminate in “ide,” such as fluorides, sulphides, etc., are simply combinations of a non-metallic element with a metal.