Short Cuts To Certainty

(via The Australian Gemmologist, Vol.12. No.2, May 1974) B W Anderson writes:

Although in the case of rubies synthetic stones show a markedly brighter fluorescence under shortwave ultraviolet light than natural stones from Burma or Ceylon, it would be unwise to rely on this effect except perhaps as a means of indicating which stones in a large parcel should be picked out as samples for examination under microscope. In a laboratory where X-rays are available a surer distinction is provided by the prolonged red after-glow (phosphorescence) shown by synthetic rubies by whatever process these are manufactured, in contrast to the almost instantaneous extinction of the red glow in the case of natural rubies when the rays are cut off. Another useful indication where small extremely ‘clean’ synthetic rubies are concerned is afforded by placing these (in a dark room table) downwards on a sheet of slow printing paper in a flat bottomed dish, accompanied by natural Burma and synthetic rubies of similar size. Sufficient water should be added to cover the stones, and they should then be exposed for a few seconds to rays from a shortwave lamp held some two feet above the dish. The paper is then developed and, if the exposure has been correctly judged, the natural and synthetic rubies chosen as standards should show very clearly the much greater transparency of the synthetic rubies to the 2537 Å mercury radiation from the lamp, on which basis the origin of the ‘unknown’ stones under the test can be judged.

The popular type of synthetic corundum made to simulate alexandrite is usually easy to recognize by eye, and shows the curved growth lines clearly. Where there is any difficulty here, the narrow absorption line at 4750 Å in the blue forms a rapid confirmatory test.

Another useful ‘shortcut’ test between natural and synthetic stones is found in the case of red spinel. When natural red spinels are exposed to a powerful beam of blue, or longwave ultraviolet, light and the resultant red fluorescent glow is examined with the spectroscope the curious fluted series of fluorescence lines, giving an ‘organpipe’ appearance, is exceedingly distinctive and forms a complete proof that the stone is natural red spinel. Synthetic red spinels, though not common, have been made by the Verneuil and other processes, and also show a bright red fluorescence. But the spectroscope here reveals no organpipe structure but a strong line at 6850 Å with a fainter band on the shortwave side of it, giving very much the appearance of the ruby fluorescent spectrum, though the wavelengths of course are different.

Turning now to other uses of the spectroscope in providing a rapid and complete testing method, one may mention the case of white or blue or golden zircons, all of which have been heat treated in Bangkok or elsewhere. Unlike Ceylon zircons which are well-known to show a strong series of absorption bands, the lines from these heat treated stones are very narrow and faint may even be missed in transmitted light. But I have found that the strongest of the absorption lines, that a 6535 Å in the orange-red, can always be seen by internally reflected light, together with the weaker line at 6590 Å which is its companion, these two lines being completely diagnostic for zircon. This rapid test is particularly useful in confirming the presence of small rose-cut zircons simulating diamond in the surrounds of rings or brooches.

Amongst the many other distinctive absorption bands which provide valuable ‘shortcuts’ in testing, I should like to mention one other which we have found particularly useful. This I the narrow and intense band at 4370 Å in the violet which is seen in light reflected from jadeite. Though this can be seen most easily in pale green and mauve varieties of the mineral, it can also usually be detected in the better quality green jadeites if a strong enough beam of blue light is used, and the slit of the spectroscope slightly widened. Incidentally, bands in the red end of the spectrum provide the surest means of distinguishing stained from unstained green jadeite.

There are so many ways in which both the appearance of diamond and its properties are distinctive that a number of ‘shortcut’ tests must suggest themselves. But the enormous importance of diamond as a gemstone, its appearance in so many forms of jewelry, and also the never ending attempts to synthesize substances which resemble it in appearance make a knowledge of variety of definitive tests advisable.

Simplest of all, of course, is a properly conducted hardness test. A polished piece of synthetic corundum should always be at hand against which an edge or corner of a suspected diamond can be carefully but firmly applied. Diamond is the only stone, which will ‘bite’ on a sapphire surface. Any mark produced should be rubbed and examined with a lens.

Extreme transparency to X-rays is another attribute of diamond which it is useful and reliable in the experience of vantage of providing visible evidence when comparison stones of known varieties and of similar size are also shown on the same radiograph.

Another unique feature of diamond, however, can only be used in stones which have a rather strong blue fluorescence under longwave ultraviolet light, is the brief yellowish after-glow shown when the stone is removed from the rays. Some practice is needed in observing this interesting phenomenon. The eyes should be dark-adapted and the fluorescing stone, held in the cupped hands, should be removed swiftly from the rays and the eye applied at once to the small dark chamber formed by the hands. There are several gemstones which show a blue fluorescence, but of these only diamond shows a yellow after-glow.

If space allowed I could describe many further ‘shortcut’ tests for other varieties of precious stones, but those I have given above are undoubtedly amongst the most useful and reliable in the experience of myself and my colleagues in the laboratory of the London Chamber of Commerce. Most of them are already well-known, but some I hope will be new to readers and help to shorten their labors when faced with testing problems which have to be solved not only with certainty but with speed.