Short Cuts To Certainty

2007: B W Anderson is a giant in gemology. This is an excellent article for students, gem traders, lab gemologists and consumers. The methodology is simple and straightforward. Many in the trade do not understand lab gemologist's problems. The trade and consumers want quick results; they want lab gemologists to make god-like statements. To detect intruders in a mixed parcel of colored stones requires insight, knowledge, analytical skills and speed. This skill will come only with discipline, experience and guerilla concentration. Today's gemologists will have to be familiar with new synthetics, treated stones, and a variety of simulants very different from the 1970s.

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

The procedure when testing gemstones in a busy trade laboratory, such as that in which I spent all my professional life, must of necessity differ radically from the adopted by a student. And this difference does not depend so much on the apparatus available as upon the purpose of the exercise. The student’s chief task is to learn; the professional’s task is, above all, to identify.

For the student it is both interesting and advisable to study each specimen in an orderly and planned manner, beginning in most cases with the determination of its refractive indices on the refractometer, and continuing with a study of its inclusions under the microscope, its absorption of light by means of the spectroscope, its dichroism, and so on. There may even be time for an accurate determination of its specific gravity. By doing all this, the young gemologist becomes accustomed to handling all his instruments, and gradually learns from first-hand the properties of each gem variety. The values he measures cease to be figures in a textbook and become recognized as living attributes of the stones concerned. If his teacher is wise he will also become trained in the intelligent use of a 10x lens, which alone can be his constant companion outside the laboratory, and with which so many gemological decisions can be made without resort to more elaborate instruments.

For the laboratory gemologists, while the first essential is of an accurate appraisal of each specimen, the next and pressing necessity is for speed. Once he has achieved a conclusive result he may have to deny himself the pleasure of further examining a stone in all its fascinating aspects, and after completing his report, must pass on to the next test. So often it seems that his client is waiting to known his decision before signing a cheque to conclude a deal.

There will usually be a wide choice of test available, but, after careful preliminary inspection (which in itself may prove conclusive) the expert must decide which test or tests will provide him with a specific answer. Since commercial gem testing so often consists in distinguishing between natural and synthetic stones, the refractometer is seldom needed, which is just as well as, however carefully handled, the soft glass of that instrument soon becomes marred by scratches and must then be repolished. A refractometer on which 1000 stones have been tested will certainly need repair while, in contrast, a micrometer or spectroscope through which a hundred times that number have been examined will have suffered no ill effects.

The nature of the goods will often dictate the first step in testing quite clearly. For example, where hundreds of small rubies are submitted it will be a valid assumption that the stones are all red corundums, the point at issue being whether they are natural or synthetic. The gemologist will therefore turn first to the microscope, knowing that in the course of examination of the stones under magnification he will, as an experienced gemologist, be able to satisfy himself as to both the origin and the species. Initially the stones can be examined in the dry state suitably distributed in lines on a glass plate. Any stones which do not at once show distinctive features can be put on one side for more thorough examination, either under oil immersion or by alternative methods.

To take another instance—a parcel of stones purporting to be tourmalines. In this case the stones would clearly be destined for a refractometer test, since here the microscope would of little value since there are (thank goodness) no synthetic tourmalines yet available, and refractive index and birefringence measurements will provide the definite answer required. In passing, one may remark that in testing such stones as tourmaline, quartz or beryl, which have a low R.I, it is very advantageous to use a Rayner ‘spinel’ refractometer, since this gives easy readings to three places of decimals in ordinary white light and suffers hardly any mechanical damage.

It is much difficult to decide a plan of action where a large parcel of mixed stones is to be tested, or a brooch or necklace set with a variety of stones, as each species may well need a different approach, and it is tedious and time consuming to be moving constantly from one instrument to another. In the case of a large parcel of mixed stones it may indeed be worth while to begin by sorting the stones by eye into groups which appear to be of the same type, after which the determination of each group will be far more rapid. Where the majority of stones in a parcel or group are all of one kind there are several ways in which any ‘intruders’ may be rapidly detected. For example, when in a parcel of yellow topaz stones some citrines have intruded, a flotation test in a bromoform solution will quickly separate them, and another useful technique is to make plain any differences in refractive index by immersion in a suitable liquid. One such case was clearly demonstrated when a contact photograph was made of a necklace consisting mainly of colored tourmaline beads. In this case the necklace was immersed in bromobenzene (R.I=1.56) in a glass dish under which a slow photographic film was placed in a dark room and exposed for a few seconds to an overhead light. The print from the developed film showed clearly a thick dark outline in each of the tourmaline beads indicating that their R.I was well above 1.56, whereas the five aquamarines which were mixed in with the tourmalines nearly matched the liquid in R.I and showed hardly any outline, making them very easy to distinguish.

Sometimes the detection of ‘intruders’ may be accomplished in a quite unorthodox manner. I can remember when some 30 years ago I had to test a packet of 117 small green stones, labeled ‘dark olivines’, which a preliminary inspection revealed as being mainly demantoid garnets. Here the most rapid and positive test proved to be examination under a low power microscope. I had recently become aware of the prevalence of ‘horsetail’ asbestos fibres as an almost inevitable inclusion in demantoid garnets, and was delighted to see these most characteristic features in all but seven of the stones. These seven were then examined with a spectroscope, which enabled me to complete the test in a very short time. One stone was a ‘clean’ demantoid, two were peridots, one a green sapphire, and three were green andalusites from Brazil of that kind which show delicate absorption bands due to manganese.

Of course, even an experienced gemologist may choose a wrong approach to a problem, and is annoyed to feel that he has wasted too much time in completing a test. Mathematicians have a word for the proof of a problem which is not only sound but also rapid and incisive. They speak of such as an ‘elegant’ proof. And it is an ‘elegant’ proof that a good gemologist would always wish to choose, and which gives him real satisfaction as a craftsman and as a scientist.

In the interests of time-saving the experienced gemologist is glad to take advantage of certain features which he knows to be unique to one species or one variety of gemstone. Particularly valuable are those tests which not only determine the species of gemstone, but at the same time prove it to be natural or synthetic in origin. Such are ‘shortcuts to certainty’ referred to in the title of this article, and I will now proceed to give details of some of these which have proved of particular value in the London laboratory.

First in importance I would undoubtedly place the group of absorption bands in the blue part of the spectrum (in particular the strongest of these, at 4500 Å) which can be seen in natural sapphires. Since C J Payne and I first observed this band in 1933 we have noted its presence in many thousands of natural sapphires but never in any faceted synthetic sapphire. We thus came to regard the presence of the 4500 Å band as a valuable and undeniably proof that the stone concerned was a natural sapphire (though nowadays the possibility of a doublet consisting in part of natural sapphire must be borne in mind). The band also indicates that geographical origin of the stone to some extent, according to the content of Fe2O3 typical for each locality. Thus while Ceylon sapphires may show only a single faint and narrow band at 4500 Å, Australian stones and green sapphires show a strong and broad absorption block within which the three main bands at 4500, 4600 and 4710 Å can be discerned. Sapphires from other localities, arranged in order or increasing absorption strength, can be written thus: Burma, Kashmir, Siam, Kenya, Montana. In cases where the band is almost invisibly weak, as is often true of Ceylon stones, it is wise to confirm it by wavelength measurement to ensure that the supposed band is not a matter of imagination or wishful thinking. The Beck ‘wavelength’ spectroscope is admirable for this purpose, since the crosswires in the eyepiece can be made to traverse the spectrum by rotating a drum marked in supposed band without the observer knowing what the reading is until he looks at the scale on the drum.

Observation is greatly assisted in this part of the spectrum if the light is filtered through a flask containing strong copper sulphate solution, and, since the 4500 Å band is an ‘ordinary’ ray phenomenon, a Polaroid disc may also serve to emphasize the reality of its presence.

The 4500 Å complex is also useful in confirming that yellow sapphires from Siam (Thailand) or Australia are not synthetic corundums, as these show the bands quite strongly. In yellow sapphires from Ceylon the band is seldom visible, but here an apricot-colored fluorescence under longwave ultraviolet light is a helpful sign (as well, of course, as the inclusions which are usually present).

Looking at the problem from the synthetic side, the common reaction of synthetic blue sapphires to shortwave ultraviolet light is to show a curious whitish or greenish surface fluorescence which is so weak that it needs a darkened room and dark-adapted eyes for its observation. Since natural Ceylon sapphires have on rare occasions been known to show a similar stringent one by examining the surface of the fluorescing stone, since this frequently reveals curved structure lines, even when these are hard to see in ordinary light.

The wise gemologist will not supersede the conventional examination of sapphires under lens or microscope, which in themselves often enable him to make a sure and quick decision, but they can be enormously helpful in providing proof in the case of ‘clean’ stones showing no easily observable structures.

Short Cuts To Certainty (continued)