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Besides
counterfeits wholly of glass, many precious stones are adulterated, so
to speak, by making a portion of a genuine stone, and employing glass
for the remainder. Such fabrications are called doublets, the upper
part of the stone being of course the genuine portion. The application
of a file to the upper and lower parts in turn will usually detect the
fraud; or if the two parts are cemented together by gum mastic, as is
usually done, they will separate on being soaked in warm water. The
union can also often be seen on holding the cemented stone up to the
light. Occasionally, however, the two are fused together, in which case
soaking would not separate them, nor would the plane of union be
visible. A desired color is sometimes given to doublets by inserting a
foil between the two portions.
Besides
the use of glass in place of precious stones, an effort is often made
to substitute a cheaper stone for the one represented. Quartz, white
sapphire, and topaz may thus be substituted for diamond, pink topaz for
ruby, and so on. In such cases the distinction of hardness is not as
marked as if glass is employed; but the test with light can usually be
made, and determination of the specific gravity, or other property,
often serves to detect the counterfeit.
Besides
employing the above-named devices, deception is sometimes achieved by
making a large stone of two smaller ones of the same mineral cemented
together. Again, inferior stones may have their backs painted to give
them a desired color. The practice of setting a stone against a foil in
order better to bring out its color or luster — as, for instance,
mounting an opal or moonstone on black, or garnet against silver — is
not considered illegitimate, and should by all means be employed when
the effect of a stone can thus be enhanced.
The
difficulties of detecting fraudulent gems will obviously be greatly
increased if the stones are set. Gems should, therefore, always be
purchased loose if possible, especially costly ones.
On
the whole, the accurate distinction of gems, or detection of frauds,
requires knowledge of the different physical characters of each
species, such as hardness, specific gravity, and behavior in polarized
light. A single test is rarely sufficient to identify a gem; but by the
use of several, perfectly trustworthy results can be obtained.
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