DOUBLET || DOUBLING. The sapphire may be counterfeited with very bright Prussian blue, mixed with a little of the above-mentioned crimson lake, to give it a cast of the purple. The Prussian blue should not be very deep coloured, or but little of it should be used: for otherwise, it will give a black shade that will be injurious to the lustre of the doublets.
The emerald may be well counterfeited with distilled verdigrise, to which is added a little powdered aloes. But the mixture should not be strongly heated, or kept long over the fire after the verdigrise is added: for the colour is apt to be soon impaired by it.
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 may be 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 mixing the powdered aloes with a little dragon's blood, or by good Spanish anotto; 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, aigue 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 correspondingly to the hue of the stone to be imitated; to which end it is proper to have an original stone, or an exact imitation of one, at hand when the mixture is made, in order to the more certain adapting the colours to the effect desired: and when these precautions are taken, and the operation is well conducted, it is practicable to bring the doublets to so near a resemblance of the true stone, that even the best judges cannot distinguish them, when well set, without a peculiar manner of inspection.
There is, however, 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.