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Valuation of Gems

The unit of valuation by weight of gems in most countries is the carat. This term meant originally, according to some authorities, the weight of a bean of the coral tree (Erythrina), known in Africa as kuara, and used there for weighing gold-dust. Others believe the term to be derived from the Greek word keration, said to be the name of the fruit of a variety of acacia having seeds of remarkably uniform size. As at present employed the weight of the carat expressed in grams is about one-fifth of a gram (200 milligrams), but varies in different countries from 197 to 216 milligrams.

The accepted weight in most of the large gem markets, such as Paris, London, and Berlin, varies little from 205 milligrams. This makes a carat weight a little over 3 grains troy, the exact decimal being 3.165. Hence one grain troy equals 0.316 carat, and one ounce troy equals 151.7 carats. The weight of the carat is usually given as four grains troy, but this is obviously not quite correct. The carat is subdivided into four equal parts, also known as grains, which evidently have not quite the weight of the troy grain, although the two are often confounded. The balances used for weighing gems are usually divided into sixtyfourths, and the fractional parts of a carat weight are then expressed by series of common fractions rather than by one fraction or a decimal. Thus a gem weighing 3 35/64 carats might have its weight expressed in this manner, 31f2, 1/32, 1/64. This is a record of the successive divisions of the scale met in making the weight, not reduced to a simple fraction.

The size of a stone of a given number of carats obviously varies with specific gravity of the gem; a two-carat sapphire, being a smaller stone than an emerald of the same weight.

The size of a stone, besides being indicated by weight, is frequently expressed by a number. This number refers to a scale of standard sizes adopted by jewelers, which runs from 1 to 50. Thus a stone of the size of No. 12 in the scale has a weight of one-eighth of a carat, No. 24 one-half a carat, No. 38 two carats, and so on. The scale thus affords a means of distinguishing smaller differences of size than would be convenient by weight alone.

The measurement of the weight of pearls differs from that of other gems in that pearls are measured by their weight in grains. The grain here employed is not the troy grain, however, but four-fifths of it, so that four troy grains are equal to five pearl grains, and a troy ounce contains 600 pearl grains.

So far as the more precious gems are concerned, it may be noted that their price increases in a much higher proportion than does the weight. According to a rule, sometimes called Tavernier's and sometimes Jeffries' rule, the price should increase as the square of the weight. Thus if a carat stone is worth $80, a five-carat stone would be worth not five times $80 =$400, but 52 or 25 times $80 =$2,000. The rule, however, affords no more than an approximation of the value, it giving in general too high a result. Some gems, such as amethyst, topaz, and others, increase in value only in about the same proportion as they increase in weight, since large stones of these species can be readily obtained.

In addition to weight, quality is a factor largely affecting the price of precious stones. To be of the first quality, or first water, a gem- must be of uniform luster and color, must be free from cracks of every kind, from bubbles, and if transparent, from inclusions of every sort, cloudy spots, or streaks. Any of these flaws can usually be distinguished by holding the stone between the eye and the light, or they are more clearly brought out if the stone is immersed in a liquid with high refractive power, such as oil of cloves, linseed oil, or even kerosene. These flaws may occur in the rough stone, or the operation of cutting may produce little cracks, called feathers, which injure the value. Obviously, therefore, to be sure of obtaining a flawless gem, it should be purchased after the operation of cutting has been completed.

The value of rough stones compared with those cut varies with the variety and size of the stones. Not only does cutting reduce the stone in size, but the cost of cutting must be taken into consideration. The latter may represent almost the entire value of stones the raw material of which is abundant and cheap, as is true of many of the varieties of quartz. In the case of diamonds the cost of cutting adds about 50 per cent to their value.