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A type of imitation gemstone that is a composite stone made of three layers, two layers being of stone with an intervening layer at the girdle being coloured cement or gelatin that provides or enhances the colour. Some such stones have the outer layers of slices of the identical gemstone, e.g. emerald of pale colour, or beryl, or rock crystal or opal.
In some cases the lower layer is an inferior stone or glass. A star sapphire triplet is sometimes made by having a star-stone of rose quartz cut en cabochon cemented over a layer of blue glass backed by a mirror (the mirror being needed because the star in rose quartz is seen only by reflected light). All such triplets are sometimes loosely referred to as a doublet.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson