It has been the practice for centuries to build up gemstones by fusing
or cementing a shaped piece of natural gemstone to another piece, or
other pieces, of inferior or artificial material.
A colorless common beryl crown cemented to a pavilion of green glass
produces an emerald doublet—part natural, part artificial—of good color
and high durability. A thin piece of beautifully colored opal cemented
to a base of inferior opal provides an assembled stone that looks like a
thick piece of high-quality opal. Triplets, and even stones in which
there are pockets of colored liquids or metal foil between the shaped
pieces, are known.
Usually, assembled stones are easily detected, since the joint will show
under magnification, but sometimes they are mounted in settings that
obscure the joint, and detection is more difficult.
[Illustration: Assembled imitation gemstones. If it were measured on
its natural ruby table, the assembled stone shown at top would have
all the characteristics of a large ruby, including refractive index.
The color of the quartz and glass combination (middle) depends on
the color of the liquid in the cavity. Since emerald is green beryl,
an inexpensive colorless beryl sandwich of green glass (bottom)
would appear to be an expensive emerald. The joints of assembled
stones often are hidden in the jewelry mountings.]