DOUBLET, among lapidaries, implies a counterfeit stone, composed of two pieces of crystal, and sometimes glass softened, together with proper colours between them; so that they make the same appearance to the eye as if the whole substance of the crystal had been tinged with these colours.

The impracticability of imparting tinges to the body of crystals, while in their proper and natural state, and the softness of glass, which renders ornaments made of it greatly inferior in wear to crystal, offer inducements to the introduction of colouring the surface of crystal wrought in a proper form, in such a manner that the surfaces of two pieces so coloured being laid together, the effect may appear the same as if the whole surface of the crystal had been coloured. The crystals, and sometimes white transparent glass so treated, were called doublets, and at one time prevailed greatly in use, on account of the advantages with respect to wear which such doublets had, when made of crystal, over glass, and the brightness of the colours which could with certainty be given to counterfeit stones in this way, when coloured glass could not be procured, or at least not without a much greater expense. Doublets have not indeed the property which the others have, of bearing to be set transparent, as is frequently required in drops of ear-rings and other ornaments; but when mounted in rings, or used in such a manner that the sides of the pieces where the joint is made cannot be inspected, they have, when formed of crystal, the title to a preference over the coloured glass; and the art of managing them is therefore, in some degree, of the same importance with that of preparing glass for counterfeiting gems, and is properly an appendage to it, as being entirely subservient to the same intention. The manner of making doublets may be shortly described.

Let the crystal or glass be first cut by the lapidaries in the manner of a brilliant, except that, in this case, the figure must be composed of two separate stones, or parts of stones, formed in the manner of the upper and under parts of a brilliant, if it was divided into a horizontal direction, a little lower than the middle. After the two plates of the intended stone are thus cut, and fitted so exactly that no division can appear when they are laid together, the upper part must be polished ready for setting; and then the colour must be put betwixt the two plates by the following method. Take of Venice or Cyprus turpentine two scruples; and add to it one scruple of the grains of mastich choseri perfectly pure, free from foulness, and previously powdered. Melt them together in a small silver or brass spoon, ladle, or other vessel, and add to them gradually any of the coloured substances mentioned below, but well powdered; stirring them together as the colour is put in, that they may be thoroughly commixed. Then warm the doublets to the same degree of heat as the melted mixture, paint the surface of the lower part, and put the upper one instantly upon it, pressing them to each other, but taking care that they may be conjoined in the most perfectly even manner. When the cement or paint is quite cold and set, the redundant part of it, which has been pressed out of the joint of the two pieces, should be gently scraped off the side, till there be no appearance of any colour on the outside of the doublets; and they should then be skilfully set; observing to carry the mounting over the joint, that the upper piece may be well secured from separating from the under one.

The colour of the ruby may be best imitated by mixing a fourth part of carmine with some of the finest crimson lake that can be procured. The sapphire may be counterfeited with very bright Prussian blue, mixed with a little crimson lake, to give it a cast of the purple. The Prussian blue should not be very deeply coloured, or but little of it should be used, otherwise it will give a black shade that will be injurious to the lustre of the doublets. The emerald maybe well counterfeited with distilled verdigris, to which is added a little powdered aloes. But the mixture should not be strongly heated, or kept long over the fire after the verdigris has been added. The resemblance of the garnet may be made with dragon’s blood; which, if it cannot be procured of sufficient brightness, may be helped by a very small quantity of carmine. The amethyst maybe imitated with the mixture of some Prussian blue with the crimson lake; but the proportions can only be regulated by direction, as different parcels of the lake and Prussian blue vary extremely in the degree of strength of the colour. The yellow topazes may be counterfeited by mixinig the powdered aloes with a little dragon’s blood, or by good Spanish arnotto; but the colour must be very sparingly used, or the tinge will be too strong for the appearance of that stone. The chrysolite, hyacinth, vinegar garnet, aqua-marine, and other such weaker or more diluted colours, may be formed in the same manner, by lessening the proportions of the colours, or by compounding them together corresponding to the hue of the stone to be imitated; for which purpose it is proper to have an original stone, or an exact imitation of one, at hand when the mixture is made, in order to adapt the colours to the effect desired.

There is an easy method of distinguishing doublets, which is only to behold them betwixt the eye and light, in such a position that the light may pass through the upper part and corners of the stone; when it will easily be perceived that there is no colour in the body of the stone.