A composite stone is two or three pieces of material fused or joined by colorless cement. Although any stone may be made into a composite, opals are the best-known. An opal triplet consists of a piece of good opal sandwiched between a top layer of clear quartz and a bottom layer of low-quality opal. A doublet is usually a good opal underneath a quartz layer. The quartz helps protect the delicate opal.
Composites also allow the use of gemstones too small to be used otherwise.
False composites contain no gemstone material. False opal doublets are made from crystal cemented over abalone shell. Soude emeralds are crystal cemented over green glass. Foilbacks have been made for nearly 4,000 years, using a variety of techniques. One kind of foilback involves placing a backing of foil or a metallic coating on a stone to give it a more brilliant color.