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
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