PART IX. False Precious Stones.
Under the name of false precious stones, there arc two kinds of productions which are essentially different—the one natural, the other artificial. The first comprehends stones sufficiently hard to resist the file; they are generally quartz, either hyaline or variously coloured. The second consists of artificial compositions of the nature of glass.
There is an intermediate order, the productions belonging to which, if well executed, are especially calculated to deceive, and are used to great extent in the East Indies. They are called semi-stones, or doublets. It is of some importance to examine this subject, because there is a prevalent belief that all false stones necessarily have glass as their base, and are consequently of little hardness. People often say, when their rubies or their topazes are declared false, ” But, see, here is a file; try to scratch these stones; you will not succeed.” Very true; but submit any piece of quartz to the same test, and the result will be the same.
Since, as we have said, hyaline or variously coloured quartz is very abundant in nature, it is easy to procure, at insignificant prices, stones that perfectly resist the file, and show, often in a remarkable manner, the whole series of colours that we admire in real precious stones.
Stones of this kind are very abundant in commerce; it might be said that, with few exceptions, all those designated as occidental are of this character, and possess consequently hardly any value. Another deception of the same kind consists in passing off a stone of a certain nature and a certain value, for another stone of a different nature and a much higher value.
The colourless varieties of sapphire and topaz, which in density, in hardness, and in refractive power differ but little from the diamond, are frequently cut into roses and brilliants, and sold for diamonds. A proof of this fact is furnished by the commercial price of the colourless topaz, which is much greater than it could obtain as topaz. It is valued in the secret hope that after cutting it may be sold for diamonds.
At the present day there are means—such as the scales for determining specific gravity, polariscopes, &c.—for distinguishing with mathematical certainty the diamond from the sapphire or topaz; but these tests are of modern origin; and in the middle ages not only colourless topazes, but those whose tint had been removed in different ways, principally by the action of fire, frequently passed current for diamonds.
Nay more than this, under the influence of the ideas that then prevailed concerning transmutation, the successful experimenters believed that they had actually transformed rubies and topazes into diamonds.
Cardan furnishes some very curious details on this subject. He gives a receipt by which ” a limpid sapphire of a faint colour” may be boiled in melted gold and converted into a true diamond.
SEMI-STONES OR DOUBLETS.
This mode of imitating real stones, though varying in a great many respects, is generally effected by giving the proper shape to a morsel of strass; removing from the upper portion of it a certain thickness, and replacing this by hard stone in such a way as to complete exactly the strass stone, then mounting the whole in a setting that completely conceals the line of junction of the two stones.
Doublets are of two kinds: in both the under part is strass, but in one the upper part is a plate of the real stone; in the other, it is simply hard stone, generally quartz, and of no value. The invention of this process has been attributed to a modern jeweller of Paris, named Bourguignon; but in reality it can be traced as far back as the fifteenth century.
A complete description of the mode of manufacturing doublets is given by Cardan, who has even preserved for us the name of the inventor:— “A fraud of a very bad character, and one very difficult to find out, was employed by Zocolino.
This venerable personage used to take a thin fake of real precious stone, such as carbuncle, emerald, &c, when he wished to imitate the carbuncle or emerald, choosing such pieces as had but little colour, and were consequently very cheap. Underneath he placed a piece of crystal sufficiently thick, and united the two parts by means of a transparent glue, in which he incorporated a colouring matter in harmony with the stone that he meant to imitate—brilliant red for carbuncle, green for emerald, &c.
He concealed the line of junction of the two parts by means of the setting; and to avoid giving rise to suspicion, he set them in gold, which was not allowed except in the case of real precious stones. “In this way this magnificent workman deceived everybody, even the lapidaries.
However, the fraud was at last discovered, and Zocolino took refuge in flight. ” It appears that this personage had a peculiar disposition for fraud, for he turned his attention afterwards to the fabrication of counterfeit money; and ended by being condemned to death.”
An examination of the objects adorned with precious stones, that have been executed in the middle ages, shows that the process described by Cardan was not unfrequently employed.
FALSE PRECIOUS STONES OF THE ARTIFICIAL KIND.
The basis of all false stones of this kind is glass. A fixed alkali (soda or potash) and silica heated to a red heat will combine and produce glass. Alumina, lime, magnesia, &c, may enter into the combination with the silica; but the result in both cases is colourless, or what is ordinarily called white glass. But if to these substances metallic oxides, or metals in a divided state, are added, even in minute quantities, the result is coloured glass.