viii Additional Notes on Diamonds, &c.
Doublet or Semi-Stone, p. 218.
new species of fraud has lately been introduced by some unprincipled
persons, which consists in using a plate stone, painted at the back,
and then foiled. This is extremely difficult of detection, and the
system cannot be too severely reprimanded; for although the practice of
foiling stones of pale colour is universal, still, if the purchaser be
warned that the stone is foiled, he knows what he is buying;
but, by means of painting the stone, its natural colour is entirely
disguised. A piece of crystal may be, and often is, sold for a
sapphire, topaz, amethyst, &c.