materials
for imitative gems, but also of the fused compounds having the precise
(or at least analogous) chemical composition of various gem-stones
which have been prepared by Mr. Grcville Williams and M. Feil. The
green beryl glass of the former, and the blue lime spinel of the
latter, afford cases in point.
Instead
of substituting a wholly imitative preparation for a true stone, a
doublet or triplet is constructed, in which a colourless or pale
stone, of no value, is made to appear possessed of a fine deep colour.
The doublet sapphire has a table and crown—all the stone down to the
girdle—of colourless or pale blue sapphire, then the lower part of the
combination, attached by cement, is made from blue glass or strass. If
then the upper part of the stone be tested for hardness it answers to
that of the sapphire, but if the base be examined, it immediately
betrays its softness. To avoid this the triplet has been devised. Here
we have pale sapphire for crown and base, but a thin layer of deep blue
'glass at the girdle—a part generally hid by the mount. To detect this
imposture immersion in water generally suffices, for then the three
layers will become visible ; and if a doublet or triplet be boiled in
water, or soaked in a small bottle of chloroform, it usually betrays
its composite nature by falling to pieces. We should add that some
false stones of this sort are coloured by means of a layer of coloured
varnish or cement.
Imitation
pearls claim a word of description. They are small spheres blown on
tubes of slightly opalescent glass, and coated internally with a
preparation made from the scales of a certain fish (as the bleak), and
called Essence d'Orient. Into the little opalescent glass globe a
coating of parchment size is introduced, and then a film of the pearl
essence. Lastly, when the essence is dry, the bead is filled with wax.
In order to produce an appearance like the orient of the true
pearl the glass globes before filling are sometimes heated under
pressure with a hydrochloiic acid solution ; in this way an iridescent
surface effect is produced.
