A counterfeit gem, composed of two pieces of crystal, with a color them, and thus giving the appearance of a naturally colored gem. Also, a piece of paste or glass covered by a veneer of real stone.
A form of gemstone trickery that was devised to allow inexpensive materials to imitate the more valuable gemstones before modern synthetics were available. A doublet can take several forms but always involves a fake gemstone produced by gluing together two different materials to form an illusion. A very common one in Victorian times was the garnet and glass doublet. This involved a red garnet top, glued to a colored glass bottom. The refractive properties of a faceted stone are such that the red of the garnet only shows at odd angles, or if the stone is immersed in a special liquid with a high refractive index. Thus, for example, a green glass bottom with a garnet top will give the appearance of a fine emerald because the top is a natural gemstone with cut facets, and a few natural imperfections, and the bottom is bright green which reflects throughout the stone. The effect is hard to appreciate unless you've seen one.
There are many gem fakes, one of them is this. In it a thin, flat section of a real gem is pasted atop a thick base of glass or rock crystal.