A doublet is a stone composed of two pieces cemented or fused together, often of two different materials. The term triplet is sometimes applied to that form of doublet in which the color is imparted to the stone by the use of a colored cement to join the two parts, and also to a stone consisting of three separate parts. The term foilback describes substitutes made by affixing an opaque substance, usually a metal foil, to the back of either a genuine or imitation gem material. Foil-backing serves one of two purposes, or both: it increases reflection from the back facets, thus making the stone appear more brilliant, or it imparts color. The term assembled stone is sometimes used as the generic term for any type of doublet, triplet or foilback. A coated stone is one that acquires its color from an artificial, nonmetallic coating applied to all or part of the stone.
The period of greatest importance for doublets and triplets was in the era before inexpensive synthetic stones became available. At that time, the most common doublet was that made by fusing glass to a garnet crown and then polishing the stone. Since the advent of the synthetic in quantity and in a wide variety of colors, the doublet's two major uses have been as an inexpensive substitute for emerald and to lend strength to a very thin piece of opal. The high price of synthetic emerald, compared to synthetic corundum and spinel, has continued the need for an inexpensive emerald substitute, since neither synthetic corundum nor spinel is made in a satisfactory emerald color. The so called synthetic birthstone rings are offered in such a low price range that the stones used must cost well under $5 each.
Formerly, the various kinds of doublets and triplets were classified on the basis of the nature of the material used in the composite, but there seems to be little need for this. For example, a triplet consisting of two pieces of colorless beryl with green cement to imitate emerald was called a genuine doublet, the theory being that the composite was made up of two pieces of the stone being imitated. However, the beryl used is not emerald and the name is therefore a misnomer. Two pieces of diamond joined to make a larger stone, or two piece s of opal joined to make a more durable stone, are examples of genuine doublets. Such names as semi genuine doublet, imitation doublet, false doublet and others were used. It is felt that little is accomplished by such an exhaustive classification. The present tendency is merely to identify the parts, since there is little difference in value, for example, between the parts of a beryl-and-beryl triplet and a quartz-and-quartz triplet.