Besides being employed for jewellery, precious stones were made use of by the mediæval embroiderers to increase the effect of the coloured materials and gold thread in the decoration of their robes. But when we bear in mind the accurate descriptions given by Theophilus in his _Diversarum Artium Schedula_ of the process of making false gems, it is only reasonable to assume that many of the so-called jewels were not in fact real gems, but imitations. Certain it is that in mediæval times the counterfeiting of precious stones was very largely carried on, while many accounts are preserved in early records of fines and other punishments inflicted on dishonest traders in gems who attempted to dispose of spurious stones, usually set in finger rings. In England and France during the fourteenth and fifteenth centuries it was customary for the jewellers' guild of each town to have a rule prohibiting its members from setting paste gems in real gold or real gems in plated metal; from mounting Scottish pearls with those of the East; or mingling coloured glass, or false, with precious stones. As in earlier periods, a crystal or a colourless paste was made to imitate a coloured stone by backing it with a foil. At South Kensington an example exists, set in a gold ring of sixteenth-century German work (No. 1206-'03), of a white crystal, which is cut _en cabochon_ and backed with a red foil, and bears a striking resemblance to a carbuncle (Pl. XXIII, 17). Many books on precious stones, both old and new, give receipts for the manufacture of imitation gems, made of flint glass and coloured with oxides according to the originals they are intended to counterfeit. Apart from these are false gems produced with really fraudulent intent. Since imitation stones cannot resist the file, it is the practice, besides backing a crystal with coloured foil, to back a thin layer of genuine stone--intended to resist the test when examined for hardness--with a layer of glass coloured as required. Another process of fabrication consists of placing a layer of glass between two layers of true stone. The place of the join in the "triplet" is hidden by the collet of the setting, and the deceit can only be detected by unsetting the stone and soaking it in chloroform. Another means employed for changing and improving the colours of stones is by heat, for the colour of nearly all gems is affected by heating.