Gem Testing

(via The Journal of Gemmology, Vol.10, No.1, January 1966) A E Farn writes:

The stone in question was oval, mixed cut, set in a gold ring—the stone was about 1.75 carats in size. Viewed through the microscope I could see angular zoning of color with a small crescent-shaped feather and on the surface of the stone small zig-zag erupted fractures known as shatter or chatter marks (and doubling of the back facets).

So far as I was concerned, the test was complete. Here was a typical natural blue Ceylon sapphire. My good friend, at home in his own (very efficient) set-up, would have taken the refractive index first. Nothing wrong in doing that, of course, provided you only use your refractometer once or twice a week and there is plenty of time to clean and put it away, etc. But if you test a dozen different pieces of jewelry of all shapes, sizes and categories in a morning between the opening of the registered post and lunch time, you may well forget (we are all human) to wipe off the liquid. You may also (and I have) leave the stone on the refractometer when answering the telephone and later search frantically for a lost stone—to find later, as a result, crystallization of sulphur crystals on the soft glass prism of the refractometer. If in business you suffer no interruptions—you are obviously not doing much business. It is the unavoidable interruptions, the imperious note of the telephone bell (someone wanting to know how much it will cost to test a string of pearls is the usual—and the answer can take time). Even shopkeepers who are busy are guilty of time wasting. Witness the retailer who is offered a pearl necklace to value—he hasn’t a clue but telephones to find out how much it would cost to test.

In the middle of testing a cluster calibré ruby setting to a brooch—the telephone rings, you lose your place and then the customer starts. Usually they have not counted the pearls, they are not conversant with grain size, they did not realize it would possibly have to be cut if genuine, X-rayed if cultured. They cannot give you a lead and are appalled by the charge because they do not realize till then how much is involved. Having courteously dealt with the customer one returns to the brooch and commences again. None of this matters very much provided that along the line of stones inclusions are seen and continuity of testing takes place and you can say that all of the stones you tested are in fact genuine rubies, or synthetic rubies, as the case may be. But how about that one clean stone which gave away nothing? It was a ruby because one saw doubling of the facets and shatter markings—it looked a slightly different red to the others, but because of the nature of the mounting little else other than a vertical sighting in a stone of total diameter under 2mm could be obtained. So here you are and the customer is calling back in a quarter of an hour for the brooch and you have had a lovely time answering the telephone to a probably non-productive caller.

This is not the time for one’s friends to suggest that your methods are not ideal. These occurrences cause certain delays with which our gemological enthusiasts do not have to contend. Gemologists usually deal with loose stones of reasonable size with nice flat facets. Our testing is usually in second-hand jewelry—seldom at its pristine best, with worn facets usually and if the mount is open at the back it is usually clogged up with a fine debris resulting from the onset of talcum powder, cold cream, hand lotion, soap and all the rest of the lotions, potions and detergent deterrents with which the modern woman’s hands are often in contact. I have never had a second-hand piece of jewelry sent in which has been cleaned by the sender for the purpose of testing.

Another béte noir can be the customer who is very important (and knows it) and likes immediate attention, and can hardly bear to wait. I have had such persons who bring in, say, a ruby ring for testing. They are usually very shrewd judges of color and have bought a ring and spent a considerable time testing it in their own office, only to be baffled by perfectly clean stone. What they see tells them it is real but reluctantly they have to have a laboratory test. They then expect some immediate magic in ten seconds—as soon as it is held under the microscope they ask, ‘Is it OK?’ even before one has focused the thing. However, life is not all like that, but most of our customers like to call at least next day for their goods (tested of course).

Having said all this, it now behoves me to settle down to pointing out that despite all the know-how and gadgetry available we cannot always give a definite result while the stone is in a setting. This may seem a little feeble, but in actual fact we seldom ask to have a stone taken from its setting and if we do we usually state our opinion beforehand in order not to appear wise after the event.

Mostly, when we ask for a stone to be taken from a setting, it is a very small synthetic corundum where the curved striae (if any) are running parallel to the girdle and setting. Other difficult cases can be backed baguette colorless stones in a sunken setting precluding refractometer work—these are quite a trial to prove without any doubts lingering.

At one time when we had a colorless cluster surround to a colored center in brooch or ring, we could safely say that the colorless stones were not diamond, and usually the customer was not further interested, since money matters.

Nowadays, we usually get asked what the colorless stones are, and surprisingly quite a large number of colorless/white sapphires in Ceylon jewelry are natural sapphires, which rather goes against the usual run of colorless sapphires, which are usually synthetic.

Quite recently we had a pale-pink stone set in a very ordinary 4-claw gold ring. No shoulder stones—jut a straight-forward native-cut, slightly lumpy, rounded, cushion-shaped stone. At first glance it could be a fancy spinel, a tourmaline or perhaps a pink topaz. Doing the job the wrong way round, according to my learned friend, I looked at the stone through the microscope. Except for doubling of the back facets—a suggestion of a DR of about 0.009—that was all. It could not be topaz, since topaz does not easily or readily show DR. It was not tourmaline, because the birefringence was too small, and it did not quite have the typical color of synthetic or natural pink sapphire. A horrid thought crossed by mind—taafeite. Fortunately for the peace of the laboratory, it did in fact yield a very positive DR for sapphire. My friend, who did not like my method of approach, would have been vindicated by this since he would have put it on a refractometer first of all. Unfortunately, I have a fetish for trying to pin things down by look, color, heft, and then microscope to find DR, inclusions, dichroism, shatter-marks, etc. This pink stone ring set me back a few minutes in probing, but now I had to get started and do the obvious, test for synthetic or genuine. Back to the microscope and a dish of methylene iodide. The stone was very clean; in fact, after quite a lengthy session of turning and turning the stone in ring, lowering the condenser, closing the diaphragm and doing all kinds of useful maneuvers, I cam to a full stop. The stone was a clean pink sapphire with no sign of curves or bubbles, or any feature whatsoever appertaining to natural. After half an hour of concentrated study under ideal conditions, refiltered liquid, cleaned eyepieces, objectives and mirror, I changed my mind several times and then gave it up from a microscope point of view.

One test we often use as a subsidiary is the well known phosphorescence after fluorescence under X-ray of synthetic ruby and pink sapphire. Here, at least, we would get a lead—but no, the stone was completely inert. So it could be genuine. Unfortunately the phosphorescence was only useful to confirm a synthetic whereas the reverse was not so. Our other refinement is to take an immersion contact photograph, hoping, as is so often the case, that structure-lines not readily visible to the naked eye would be revealed by the sensitivity of the film. Once again the result was negative. Somewhat reluctantly we telephoned the customer to ask him to take the stone out.

After the stone was taken out, we found a slight indication of ‘treacle’ and I think, if my memory serves me, a few lines of silk. This was not a stone suitable for the diploma examination. We returned it to our customer finally certified as genuine.

Trouble seldom comes in small doses, for a short while after we had a succession of small insignificant rings with microscopic stones in the center of cluster or as cluster surrounds, and suddenly it seemed as if we were continually asking for stones to be taken out of their settings. However, when one considers we tackle hundreds of items a month of mounted stones, our record is very good.

Gem Testing (continued)