A NATURAL RUBY can be recognized and also separated from the other GEMSTONES which to a certain degree resemble it.
Corundum is simply crystallized alumina (Al2O3) and, when in its purest form, is entirely devoid of color. The rich hue of RUBY
is chiefly due to small amounts of chromic oxide, which is able to replace part of the alumina without disturbing the trigonal crystal structure. The source for the world’s finest RUBIES has always been the MOGOK district in BURMA, where so many other interesting gem minerals are found. RUBIES from Thailand, owing to the presence of traces of iron, tend to have a less attractive color, resembling Garnet, and are not popular in the trade; while RUBIES from SRI LANKA and MADAGASCAR are often more pink than red. Nevertheless, fine RUBIES from these less-regarded districts are occasionally found, and should be sold as such, on their merit.
A more recent prolific source of RUBIES is East Africa (Tanzania) where large transparent pieces showing little crystal from have been found in surface deposits accompanied by garnets of very similar appearance. The colour of these African rubies is very pleasing, though being due as much to iron as to chromium the hue is not rich as in those from Oriental sources. At least the new locality has provided rubies of important size at a reasonable price. Large RUBIES of good quality from the better-known localities are so rare as to be far too costly for all but the very wealthy.
There is another and quite different deposits of RUBY in Tanzania where the mineral occurs in well-formed hexagonal crystals often of considerable size, accompanied by a bright green zoisite and a dark amphibole mineral. These crystals are for the most part opaque crystals from which small GEM RUBIES resembling dark, very chrome-rich BURMA material have been successfully cut.
RUBIES of good quality from other sources – Kenya, Nepal, Afghanistan, and Pakistan – have been discovered and mined in recent years, but political difficulties have often prevented a regular supply of these stones from reaching the main markets. Dealers in the trade are mostly conservative, and prefer to handle stones from recognized sources that have an established reputation. At their best, the RUBIES from Kenya approach most closely the rich blood-red stones for which BURMA has always been so famous, and the colour may even show something of the ‘treacly’ distribution that has always been recognized by GEMOLOGISTS as a hallmark of BURMA RUBIES. The Pakistan stones mostly have a multitude of inclusions and the color is not so rich, but apparently these are very iron-free, with the result that the stones may show a distinct phosphorescence after exposure to X-rays, which can be disquieting for those who have always regarded this as a useful diagnostic feature for synthetic rubies made by the artificial process.
The properties of a NATURAL BURMA RUBY may be briefly stated as follows: hardness, 9 (next to diamond on Mohs’ scale); refractive indices, 1.765 and 1.773; double refraction, 0.008; SG, 3.99 to 4.00. The dichroism is quite strong, the twin colours being pale yellowish red and deep carmine, and the absorption spectrum is highly diagnostic notably by reason of the narrow bright line (doublet) seen in the deep red and the narrow dark lines in the blue when the stone is powerfully illuminated. The former is due to the bright red fluorescence of RUBY, but the distinction from synthetic RUBY, which also has the properties so far mentioned, has still to be considered. Apart from this, the materials most likely to be confused with genuine RUBY are Spinel, Garnet and Tourmaline. A careful refractometer reading using either sodium light or a good red colour filter is in itself conclusive, since the only red stone in which the refraction is similar is Garnet.
Only pyrope Garnet, with a single index between 1.74 and 1.75 approaches the best ruby in colour, though other garnets of the almandine-pyrope series, having higher indices, may be confused with Thai RUBIES where colour is the only guide. Perhaps the simplest test to apply, though not so conclusive, is the test for dischroism, remembering always that it is essential to hold the GEMSTONES and examine it from different angles to get the strongest effect. Spinel or Garnet, will show no trace of dichroism, and the only dichroic red stone apart from RUBY is the RUBELLITE variety of
Tourmaline, which seldom, if ever, is seen in the same shade of red as CORUNDUM. The dichroic colours of red Tourmaline, dark red and pink, are, however, rather similar to the colours seen in BURMA (and synthetic) RUBY.
Another aid to the recognition of RUBY is its appearance under the Chelsea colour filter. Viewed through the filter, under a bright light, RUBY shows a peculiar vivid fluorescent red, and this, combined with its dichroism, should leave little doubt in the mind of the observer, even if a refracto-meter is not available.
The red fluorescence of RUBY is a spectacular sight under ultra-violet light (quartz-mercury lamp with Wood’s glass filter) – but visible green, blue, or violet light will also stimulate this fluorescence in ruby; hence the appearance of the bright fluorescence doublet when examining the spectrum of the stone, and its fluorescent red appearance when viewed through the Chelsea colour filter. A splendid exhibition of the fluorescence of RUBY and synthetic RUBY is provided by viewing specimens between ‘crossed filters’. None of the blue light can pass through the filter, but the red fluorescent glow from RUBY most certainly can: stones are thus seen glowing like coals against a dead-black background – a sight so beautiful that it still delights the author after years of repetition. Red SPINEL shows a bright red fluorescence under the same conditions, which to the unaided eye is quite indistinguishable from that of RUBY. The spectroscope, however, enables the two species to be separated with certainty.
In RUBY, the red glow is seen to consist almost entirely of light from the strong doublet at wavelengths 694.2 and 692.8 nm, which appears as a single line in a small prism spectroscope.
In SPINAL, there appears a whole group of bright lines, rather like a set of organ pipes, with the two strongest in the centre of the group. This ‘organ pipe’ fluorescence is a very sensitive test for spinal. No other red stones,Garnet, Zircon, tourmaline, nor any red pastes or doublets, show any red fluorescence.
The presence of iron has an inhibiting effect on fluorescence, and for this reason the red glow from Thai RUBIES is, on the whole, less brilliant than in those from BURMA. It is less generally realized that if a RUBY is too chrome-rich this also lessens the strength of its fluorescent glow. This applies to some of the BURMA RUBIES, which are often mistakenly classed as Thai RUBIES on this basis.
In some of the large East African RUBIES, though not in all, the fluorescence is very dim and the absorption spectrum only shows the chromium lines very weakly. In such stones iron probably plays as large a part as chromium in producing the rather subdued red colour.
Even is we have assured ourselves that the GEMSTONE examined is indeed a RUBY, we have still the important question to answer – is it a NATURAL RUBY or a synthetic stone? – Since in all that physical properties already mentioned the synthetic ruby does not differ from the natural. The whole question of the discrimination between NATURAL RUBIES and their Synthetic Counterparts manufactured by inverted blowpipe process. This is a big problem which constantly worries the JEWELER as it is a difficult process on examining each and every GEMSTONE in the inventory. The reputation depends on it and even a single mistake or carelessness can be fatal for the reputation. So, it is very important to be able to distinguish between the Natural and Man Made GEMSTONES if one wants to keep the reputation.
In the first place one must remember that flawless RUBIES of rich color and 3 carat plus size are, in nature, extremely rare. This fact alone should make one suspicious of a large clear specimen of RUBY GEMSTONE.
In most cases the mere appearance of the stone is enough to warn the trained observer that a stone is synthetic. This is partly due to a slight but real difference in the nature and proportions of the coloring oxides present in the stone, but is perhaps mainly due to the fact that a Synthetic RUBY is seldom cut in such a manner as to display the best color. The most attractive colored in RUBY is seen in the direction at right angles to the basal plane of the crystal.
Thus in a properly cut RUBY strong dichroism should not be observable directly through the table facet when the stone is viewed with the dichroscope. In a synthetic GEMSTONE, due to the shape of the original silk, the direction of cutting is almost invariably wrong, so that the fine purple-red of the ‘ordinary ray’ is diluted with the unpleasant yellowish-red of the ‘extraordinary ray’ and the two tints can be seen side by side in the two images of the dichroscope window looking directly through the table facet.
Another point to realize is that a NATURAL RUBY is hardly ever quite ‘clean’, which is to say that it almost always encloses small crystals of other minerals in the form of pale angular grains, cavities of irregular shape, often relatively large, and patches of fine crisscrossing canals, or of fine reddish rutile needles, which give a silky effect by reflected light and in consequence are known as ‘silk’. When the presence of either of these forms of inclusion can be detected with a lens there can be no doubt that the stone is genuine. Typical BURMA RUBY inclusions look like silk and liquid filling. Care must be taken, however, not to confuse the crystalline inclusions mentioned with the spherical or elongated gas bubbles which are often a feature of synthetic stones. These are seldom so large in size as the crystals enclosed in BURMA RUBY, and under the lens are more likely to appear as little clouds of dust-like particles. Sometimes a dense cloud of microscopic bubbles may cause a milky reflection not unlike the ‘silk’ seen in natural stones.
The distribution of color is also revealing: curious feature of the natural BURMA RUBIES is the presence of patches of deeper color in the form of wisps and swirls to which the name ‘treacle’ has been aptly applied. A hint of ‘reacle’ in a RUBY used to be a sure sign of its genuine origin but nowadays similar structures may occur in some flux-frown RUBIES. Certain other stones owing their color to chromium sometimes show a rather similar effect – e.g. RED SPINEL, pyrope, and EMERALD. All these internal features can be recognized with far greater ease and certainty under even the simplest form of microscope, which in addition opens up a whole new world of interest and beauty, for indeed the inclusions in most GEMSTONES, and particularly in RUBY, are both extremely interesting and beautiful. It is great assistance in studying internal features to immerse the specimen in glass-bottomed cell containing a highly refractive liquid such as I-bromonaphthalene. If the GEMSTONE is a synthetic free from bubbles one must rely on finding the curved line, and this may entail examining the stone out of its setting at several angles until the lines become visible. The curves are usually more clearly seen when the lighting is not too brilliant, and mirror of the microscope should be tilted to get the most marked effect, and the condenser lowered.
In Synthetic RUBIES, the curved lines are much more pronounced and by no means rigidly parallel. For comparison, a photo micrograph of a small BURMA RUBY has only the relatively large crystal inclusions but also straight lines of zoning parallel to the hexagonal outline of the original crystal. Sometimes only very small inclusions may be present, and these may easily be mistaken for bubbles by an inexperienced worker. In all difficult cases it is wiser to submit the stone for a laboratory test.RUBIES from Thailand can, on occasions, be remarkably ‘clean’, but their lack of fluorescence under X-rays and short-wave ultra-violet is then a revealing feature. The most typical inclusions in Thai RUBIES are round and opaque, always surrounded by a roughly circular ‘feather’ when these are small, careful examination is needed to differentiate them from the bubbles seen in synthetic GEMSTONES. Another feature of Thai RUBIES consists of twinning planes sometimes intersecting each other.
In case of Thai RUBIES Identification was made possible by means of microprobe analysis of the composition of the mineral grains which had been exposed at the surface of the stone by controlled polishing. Some, showing a hexagonal outline and a metallic luster by reflected light, were proved to be the iron sulphide mineral pyrrhotine, sometimes known as ‘magnetic iron pyrites’. These inclusions are thought to be primary in origin, formed by the incorporation of islets of impurities during the growth of the crystal.
Synthetic RUBIES show a brighter fluorescence than NATURAL RUBIES under short-wave ultra-violet rays, and though this test should not be too heavily relied upon it can serve a useful purpose in indicating which GEMSTONES in large parcel of caliber RUBIES or in an eternity ring should be examined with special care.
The greater transparency of Synthetic RUBY to short-wave ultra-violet light is well known and provides another most valuable background test in those few cases where there is still some uncertainty after an examination under the microscope.
It has been assumed throughout that the ‘Synthetic RUBIES’ referred to were stones made by the artificial method, which has been operating in the factories of several large producers for three-quarters of a century. But 99 per cent of the synthetic GEMSTONES used in Jewelry today are still manufactured by this inexpensive process, the growing band of crystal growers in the world today has succeeded in producing laboratory-made RUBIES that tax the skills of even of an experienced laboratory GEMOLOGIST to determine. Most of these are crystallized by the ‘flux-fusion method of growth and contain liquid ‘feather’ inclusions. Rubies have sometimes been based on small seeds of NATURAL BURMA RUBIES, which, of course, show the typical natural features of these well-known GEMSTONES. The border line between the seed fragment and the flux-fusion growth can best be seen under dark-ground illumination under the microscope. They use fluxes which include lead oxides and/or fluorides, and boron oxides, plus, of course, the necessary alumina and its colouring agents such as chromium oxide. Temperatures such as 1300 degree C are countered by the use of platinum crucibles. These fluxes are commonly included within the fabric of the RUBY or SAPPHIRE crystals and give a very useful clue as to the method of manufacture. The flux may take the form of fingerprints of feathers which resemble those seen in natural stones, but on close examination these feathers, or tubes or tiny dashed lines are seen to be filled with solid flux, sometimes with gas in the interstices of the cavities, but not with liquid. The flux may be white in color and yellow or orange flux is often seen in kashan RUBIES. Flux-grown RUBIES may show growth lines, and these may be very close together and possibly at angles which resemble the growth lines at 120 degree seen in natural Corundum. These growth lines may require careful adjustment of the lighting to see them and fibre-optic lamps and shadowing techniques are most useful in identifying growth structures in suspect Corundums. In some Ramaura rubies growth lines may be so close together that, at the appropriate angle of viewing, iridescent effects may be seen. Examples of inclusions seen in flux-grown rubies are shown in figure and Plates 6-8.
In some rubies grown by the Czochralski pulling technique the only discernible features may be faint growth striations. These very perfect rubies, used for lasers, may pose problems for the gemologist. Properties such as refractive index, specific gravity, dichroism, and absorption spectra (in the visible wavelengths) are similar for synthetics and natural rubies, but gemological laboratories with spectrophotometers may be able to detect differences in the ultra-violet or possibly the infra-red ends of the spectrum.
Such sophisticated instruments are not available to the GEMOLOGISTS, and it may be some consolation to the puzzled GEMOLOGIST to know that very experienced laboratory workers sometimes disagree on the pedigree of an apparently clean RUBY.
Recently a GEMSTONE was only categorized by the presence of a single tiny surface inclusion which proved, on electron microprobe analysis, to be margarite, a mineral which could only occur in a NATURAL RUBY. GEMOLOGISTS, however skilled, are now faced with some difficult problems in the distinction between NATURAL and synthetic GEMSTONES. One general fact may be worth remembering: in no synthetic stone does one find inclusions of ‘foreign’ minerals – that is, of minerals consisting of chemical constituents other than those belonging to the host mineral.
The only natural stone resembling BURMA RUBY at all closely in colour is the RED SPINAL, which is found in the same district. The colour in this case is also due to chromic oxide, but the tint is more brick-or orange-red then true ruby-red. In any case, the lack of dichroism in spinel and its single refraction, different absorption spectrum, and inclusions serve to distinguish it with complete certainty. When testing a parcel of RUBIES under the microscope it is surprising how obviously different in colour an ‘intruder’ SPINEL appears and how noticeable is the dichroism in RUBY.
The colour of RED GARNETS, apart from certain pyropes already mentioned, is closer to that of THAI RUBY. Garnets of this type have a very characteristic absorption spectrum containing three bands crossing the yellow, green, and blue parts of the spectrum, respectively, and this, apart from other tests, provides a very rapid and sure means of identification. Tourmaline, the only other natural stone that need be discussed in this connection, has far lower refractive indices (1.62 and 1.64) than RUBY, and a larger double refraction, which latter property will enable the practiced observer, using a lens, to detect a ‘doubling’ of the edges of the back facets of the stone if viewed through the front at a favourable angle.