Mineral Guide > Gems > Imitation Gems

Imitation Gems

The art of imitating gems has reached a high degree of perfection, and while the substitutes thus prepared have legitimate uses, the temptation to palm them off on the unsuspecting for real gems, at or near the price of the genuine, is often too strong to be resisted. It becomes important, therefore, that every one purchasing precious stones should be acquainted with the characteristics of the false as well as of the real, and unless purchasing of a perfectly reliable dealer should subject the offered stone to the most careful scrutiny.

Tourists are especially liable to deception of this sort, since their purchases must be largely made of itinerant venders, with whom they are not acquainted. The Persian turquois venders, knowing the liability of some of their wares to fade, are accustomed to leave for parts unknown as soon as their stock is disposed of, and gem-sellers of other nations often exhibit similar propensities.

Emanuel tells of a man who left his business in his own country, and at considerable expense went to England to sell a quantity of stones which he had been assured were diamonds, only to find on arrival there that they were simply quartz. This experience in one form or another has doubtless been repeated countless times, and should serve to show the importance of knowledge on the part of all purchasers of gems of the features which make them intrinsically valuable.

It may be said in general that the one quality of most gems which cannot be successfully duplicated is their hardness. The best simple protection therefore against purchase of a glass imitation for most precious stones will be found in a test of this property. Glass is softer than most precious stones, and hence is much more easily scratched than they. It will yield to the file, while they will not. This test should of course be made so as to avoid injury of the stone, for often the girdle of a gem cut as a brilliant is as delicate as a knife edge, and great care should be used in testing it.

If a file be not convenient, a fragment of quartz can usually be obtained, and affords an accurate means of testing hardness, since the hardness of quartz is 7, and that of glass rarely over 5. An aluminum pencil also affords a safe means of testing hardness. Drawn over glass, it leaves a white, silvery line, but on hard gems little if any mark. In respect to color, luster, and even specific gravity, glass may be made to imitate almost any gem. Even natural looking flaws can be made in a glass imitation by dexterous hammer blows. Nevertheless glass can often, though not always, be distinguished from a mineral by the fact that in a piece of glass minute air bubbles may be seen on examining it with a lens. These bubbles generally differ in shape and number from any found in natural minerals. Glass also has a characteristic conchoidal fracture not quite like that usual to minerals.

True gems are colder to the touch than glass as a rule, although glass is colder than such substances as jet, amber, and pearl, for which it is often substituted. The colder feeling of true gems comes from their being better conductors of heat than glass, so that they take away warmth from the hand more rapidly. For the same reason most true gems when breathed upon acquire a thicker coating of moisture than glass and lose it more quickly than does that substance.

In the application of these simple tests jewelers often become very skilful, and if the stones are not too small can pick out a diamond, sapphire, or other gem from a whole bagful of glass imitations by the above distinctions alone. When in the rough, a useful distinction of glass from most gems is to be found in the easy fusibility of the former before the blowpipe. While most gems are practically infusible in this way, glass is easily fused, and hence the trial of a splinter of the substance before the blowpipe affords a test of value.

The distinction of glass from minerals by an observation of their behavior in polarized light can be made without injury to the substance tested, and with reliable results. To be sure, the distinction of glass from diamond, spinel-ruby, or other singly refracting gem, cannot be made in this way; but when the stone is doubly refracting, as is the case with the majority of species, such investigation affords one of the surest and most convenient means of identification.

The glass used for making imitation gems is usually one having a high percentage of lead in its composition. The lead makes it soft, but gives it great brilliancy. The glass is usually known as paste, or strass, the latter name being from the inventor, Strass of Strassburg, who invented the mixture during the seventeenth century. Uncolored it affords a good imitation of the diamond, and when colored with various metallic oxides, remarkably accurate likenesses of different gems can be obtained.

Besides counterfeits wholly of glass, many precious stones are adulterated, so to speak, by making a portion of a genuine stone, and employing glass for the remainder. Such fabrications are called doublets, the upper part of the stone being, of course, the genuine portion. The application of a file to the upper and lower parts in turn will usually detect the fraud; or if the two parts are cemented together by gum mastic, as is usually done, they will separate on being soaked in warm water. The union can also often be seen on holding the cemented stone up to the light. Occasionally, however, the two are fused together, in which case soaking would not separate them, nor would the plane of union be visible. A desired color is sometimes given to doublets by inserting a foil between the two portions.

Besides the use of glass in place of precious stones, an effort is often made to substitute a cheaper stone for the one represented. Quartz, white sapphire, and topaz may thus be substituted for diamond, pink topaz for ruby, and so on. In such cases the distinction of hardness is not as marked as if glass is employed; but the test with light can usually be made, and determination of the specific gravity, or other property, often serves to detect the counterfeit.

Besides employing the above-named devices, deception is sometimes achieved by making a large stone of two smaller ones of the same mineral cemented together. Again, inferior stones may have their backs painted to give them a desired color. The practice of setting a stone against a foil in order better to bring out its color or luster-as, for instance, mounting an opal or moonstone on black, or garnet against silver-is not considered illegitimate, and should by all means be employed when the effect of a stone can thus be enhanced.

The difficulties of detecting fraudulent gems will obviously be greatly increased if the stones are set. Gems should, therefore, always be purchased loose if possible, especially costly ones.

On the whole, the accurate distinction of gems, or detection of frauds, requires knowledge of the different physical characters of each species, such as hardness, specific gravity, and behavior in polarized light. A single test is rarely sufficient to identify a gem; but by the use of several, perfectly trustworthy results can be obtained.