DOUBLET, among lapidaries, a counterfeit stone composed of two pieces of crystal, with a colour between them; so that they present the same appearance to the eye as if the whole substance of the crystal were coloured.
The crystal or glass is first cut in the manner of a brilliant, except that the figure must be composed of two separate pieces. After the two plates have been thus cut, and fitted with the utmost exactness, the upper part is to be polished ready for setting. The colour is then put between the two plates by the following method:—To two scruples of Venice turpentine add one scruple of the grains of very pure mastich previously powdered; melt them together in a small silver spoon, add to them gradually any of the coloured substances mentioned below, in a state of fine powder, and stir them well together as the colour is put in. Then warm the doublets to the same temperature as the melted mixture, apply it to the surface of the lower plate; and put the upper one instantly upon it, and press them to each other, taking care that they be conjoined perfectly even. When the cement is quite set, the redundant part of it, which has been pressed out of the joint, should be scraped off. In the setting, the mounting should cover the joint, in order to prevent the separation of the pieces.
The colour of the ruby may be imitated by mixing a fourth part of carmine with some of the finest crimson lake; the sapphire, by very bright Prussian blue, mixed with a minute portion of crimson lake; the emerald, by distilled verdigris, to which is added a little powdered aloes; the garnet, by dragon's blood, with the addition, if necessary, of a very small quantity of carmine; the amethyst, by mixing some Prussian blue with crimson lake; the yellow
topazes, by mixing powdered aloes with a little dragon's blood, or by good Spanish arnotto; the chrysolite, hyacinth, vinegar garnet, aqua-marine, and such other weaker or diluted colours, may be formed in the same manner, by lessening the proportions of the colours, or by compounding them together so as to correspond with the hue of the stone to be imitated.