A composite stone made of two layers cemented or fused together and usually intended to appear as a whole natural stone. (The term is also applied to a triplet, made of three layers.)
There are several varieties
A composite opal is sometimes made as a doublet. Some doublets include a pocket of coloured liquid or a piece of coloured metal foil. Sometimes a cameo is counterfeit, made as a doublet, with the upper part being carved glass and the lower part chalcedony, or with the two parts of contrasting stones. Doublets can be produced as counterfeits, to be deceptive or fraudulent, but some are made legitimately under recognized trade-names, e.g. the smaryll.
Some doublets are superior to coloured glass imitation gemstones; if the upper layer is a gemstone it will resist scratching when tested for hardness. All such composite stones can be readily distinguished from whole natural stones; one simple test, usually effective, is immersion in water, which causes the join to be revealed by the different light refraction of the parts.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson