Portal logo
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
51