Aspects of this topic are discussed in the following places at Britannica.
...same material, the gem is called a true doublet; if they are different, with the crown (above the girdle) being genuine and the pavilion (below the girdle) an inferior stone or glass, it is called a false doublet. True doublets may be detected by immersion in a liquid with an index of refraction nearly the same as that of the gem fragments; the layer of cement will appear as a dark line. False...
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