Ruby fragments may be heated at high temperature to partially melt them
into a large mass that can be cut into a more valuable stone. Ruby is
the only stone that can be successfully reconstituted in this way, but
there are many other ways of tampering with natural stones to make them
more desirable.
Sometimes natural stones are backed with foil or a metallic coating to
enhance their color, to provide brilliance, or to produce a star effect.
It is said that in an inventory of the Russian crown jewels by the
Soviet Government, the ruby-colored Paul the First Diamond was
discovered to be a pale pink diamond backed by red foil. Today, some
diamonds are coated on the back with a blue film to improve their color.
Aquamarine, when pale greenish blue, may be heated in order to deepen
the blue color, and poorly colored amethyst may be heated to produce a
beautiful yellow-brown quartz, called citrine, that often is
misrepresented as topaz. By strong heating, the brown and reddish brown
colors of zircon can be changed to blue or colorless, both of which
states are unknown in natural zircon. Dyes, plastics, and oils are used
to impregnate porous gems such as turquoise and variscite, and even
jade. Off-color diamonds, when exposed to strong atomic radiation, can
be changed to attractive green, brown, and yellow colors, causing them
to resemble higher-priced _fancies_.
In the constant search for something new, gem suppliers sometimes
introduce into gemstones colors that are not always an improvement. For
example, the beautiful purple of some amethyst can be converted, by heat
treatment, to a peculiar green. Such an altered stone is marketed as
_greened amethyst_.
All of this tampering with gemstones complicates the problem of
identification, so it is a matter of serious concern to the gem trade.