If There’s A Doubt Have It Tested

2007: A E Farn's tips on gem testing is still valid today, but diamond simulants such as synthetic cubic zirconia and synthetic moissanite are the most frequently encountered stones in the gem market. In my view, today synthetic cubic zirconia is still the best + affordable diamond simulant. There are new synthetics such synthetic amethyst, citrine, ametrine + synthetic ruby, sapphires (flame fusion/flux/hydrothermal), synthetic emerald + colored varieties (hydrothermal/flux), synthetic diamond (HPHT/CVD) + treated colored stones that are giving new headaches for the practising gemologist.

(via Journal of Gemmology, Vol.IX, No.10, April, 1965) A E Farn writes:

This advertising phrase is apt and very rewarding. In the world of gem testing there were some stones which hardly needed testing, since they were impossible to copy or imitate successfully. Emeralds use to be easy—if red under a filter, it was an emerald. Black opals were once a certainty, but nowadays there are treated or carbonized opals of a very attractive appearance but which are not exactly as one would expect when a stone is termed ‘black opal’.

Possibly one of the least tested stones is the diamond. By diamond I mean transparent white diamond, not any fancy color or hue, since the detection of possible treatment is a separate and very technical problem on its own.

Diamond is the hardest known gemstone and in what one terms ‘the good old days’ an anvil was said to shattered quite readily when a diamond was tested upon it by striking. Apart from being the hardest known gemstone it also has a now well-recognized weakness, i.e. cleavage. We all know what would happen if we placed a diamond on an anvil and struck it a heavy blow.

A gem testing laboratory sees more unusual stones from hopeful jewelers than most people in this trade. Some people specialize in certain stones such as star stones and cat’s eyes, emeralds and opals, or rubies and sapphires. Diamond dealers proper seldom mix with the colored stone trade. It is interesting, sometimes, to see an obvious large synthetic ruby brought in by a dealer who usually specializes in diamonds only.

Whilst there are colored stone dealers, and diamond dealers, there are dealers who dabble in many gems including corals, pearls, ivories, etc. and seldom do these people manage to specialize in any particular one.

How much more difficult then is it for the retail jeweler who has to consider all these, together with watches, clocks, gold, silver, plated wares and repairs and estimates. Small wonder then that the jeweler who is suddenly confronted with a pearl necklace, a chrysoberyl cat’s eye ring or a fine pink sapphire in a cluster surround sometimes feels himself at loss to identify such gems.

‘Are they diamond’ ‘or is it a diamond’ is the kind of question which brings quick reaction. But, and it is a very big but, once doubt is sown in the mind then fermentation takes place and the slogan ‘If there’s a doubt have it tested’ pays its dividend.

In the laboratory quite recently we had an old cut long cushion-shape thick diamond of good quality brought in by a slightly irate though somewhat apologetic diamond dealer. I thought he wanted it weighed to settle a point one way or the other. He said ‘I want it tested’. To me, it was so obviously a diamond. I could not help pointing this out as diplomatically as possible (after all he was a diamond dealer). To my relief he immediately agreed, ‘Yes, indeed I know it is a diamond but someone has doubted it because it is an old stone.’ Thus we had the crux of the matter—a doubt. This particular case, one of a few I can recall, was outstanding in its sharpness of doubt and certainty. Others, of course, are a good deal more nebulous.

Some while ago a dealer asked me if I was interested in a parcel of rose-cut diamonds and on being shown them I suggested he had them tested. He protested that they were all old Indian stones but to me they looked like zircon—and a check by spectroscope confirmed the diagnosis.

Another dealer had a diamond and onyx eternity ring, which had been fished up from the sewers by a sewerman, and the diamonds were very rubbed indeed—no one could have said what the stones were by just looking.

It is my experience that pawnbrokers above all seem to be considered fair game to the unscrupulous. Most pawnbrokers are open not only to lend money on valuables but are very often more liable to buy jewelry from the public than many retail jewelers. Therefore more people ‘try it on’ with pawnbrokers than otherwise. I am, perhaps, specifying the pawnbroker at the moment because the average retail jeweler buys from regular suppliers mostly new goods. These in turn are obtained from manufacturers who are buying their diamonds direct from well-known sources of supply. It is not these kinds of goods I have in mind.

Quite frequently a very pleasing ring (with, say, a circular amethyst or golden quartz) is mounted in a cast setting with a cluster surround of synthetic white spinels. Very clean and newly polished it looks very pleasant because the cast is from a very good patten and perhaps the finish of the ring by the polisher has been well done. This kind of ring after being worn a little (with a little dust, or powder or soap accumulation behind it) if offered over the counter under artificial light at a reasonable asking price, can cause an error. When goods are offered too cheaply suspicion is aroused. If offered at a reasonable price—then commerce overcomes gemology (if there is any gemology).

Sometimes it happens a diamond is cut with too much spread to make it look more for the money and the stone looks a little ‘laxey’, a term somewhat similar to ‘lasque’, which is used to describe a very thin flat style of cutting from India. When a diamond is cut in a manner which is not familiar to the jeweler then he is troubled. A very tricky point arises sometimes when a stone is baguette-shape and used a shoulder stone to a ring, and is set flush with the metal. This allows very little chance of inspection, due to the mount immediately obtruding when by turning the ring the stone is examined.

Artificial lighting can mislead when quickly looking at a cluster-set ring, especially if one has approached the problem by examining the center stone and assuming a diamond cluster surround. Lack of ‘fire’ can thus be put down to general dirtiness at the back and a mental note that it will improve if the ring is cleaned. It is usually the next day, with daylight to help and a clean-up of the accumulated debris from behind the stones, that the truth becomes apparent.

By these observations it is not intended or suggested that jewelers and pawnbrokers are constantly being taken in by unscrupulous methods, because the majority make their living adequately enough to disprove any such opinion.

With the advent of strontium titanate, however, I think it to be a little dangerous to assume too readily that diamonds never need testing. One afternoon, about 2 years ago, I had a strontium titanate single stone ring in to test from a West End jeweler. Immediately following we had a cluster ring from the National Association of Goldsmiths to test for one of their members. The center stone was a strontium titanate with a cluster surround of reasonably good quality diamonds. The whole effect was good and if a little dirtier could have passed as a very fine all-diamond cluster. Two rings in immediate succession one afternoon having strontium titanates in them was remarkable.

‘Diamonds are forever,’ as a title, indicates the inherent hardness and durability of diamonds. This hardness of diamond is its important factor—it is related to the quality of polish the stone can take, and upon the finish of the facet edges. In fact, a hard look and finish. Because of its simple chemical and molecular structure diamond has a single refraction and a characteristic optical clarity. Such is this impressive quality that when viewed through the table facet with a lens the culet seems to be very close to the table.

The stones which are most commonly used in jewelry in place of diamonds or as diamond simulants are: (1) synthetic spinel, (2) synthetic sapphire and natural colorless sapphire, (3) zircon, (4) synthetic rutile, (5) strontium titanate, (6) paste, with high refractive index. There may be others—one can cite almost any colorless stone, but I think the stones listed are the most commonly used and reasonable to expect.

Jewelry which is ‘diamond’—set, if suspect, should be cleaned in order that information can be obtained by visual methods. A clean stone or stones in settings are much more easy to test if light can readily be transmitted.

In a cluster setting synthetic spinels are very quickly identified by immersing in methylene iodide because their refractive indices very nearly match at 1.728 and 1.74 respectively. The effect can be quite startling, the stones tend to disappear from their settings an empty mount is left.

Since natural spinels do not exist as colorless stones (they always draw a little color in comparison to a parcel of diamonds or synthetic spinels), it is safe to assume the synthesis of the spinels. Synthetic sapphires, as opposed to diamond, are doubly refracting (as shown by a doubling of the back facets or the effect seen when a piece of jewelry is revolved between fixed crossed polaroids). Although it is safe to assume that all spinels which are colorless in a setting are likely to be synthetic it is not the same case with colorless sapphires.

Usually, of course, one is not trying especially to identify the suspect colorless stone—only to avoid buying it as a diamond. Natural colorless sapphires are very bright stones and it is not unusual to find them in cluster settings surrounding a genuine blue sapphire in jewelry emanating, say, from Ceylon. Examination by microscope will sometimes reveal crystalline inclusions, feathers, silk, etc. in even very small stones.

Synthetic colorless sapphires will very often show included gas bubbles much more readily than synthetic spinels. A thorough cleaning of the backs of small cluster-set jewelry is really important when examination has to be made by microscope. Being a cheap product synthetics are very seldom well cut or polished and reveal this by certain small useful factors such as fire or chatter marks on the new face. These marks are caused by heating due to pressures in polishing and are seen as slight surface cracks in a slightly zig-zag manner. They are more frequently seen on synthetic stones than on genuine ones.

Colorless zircons, because of their superior fire are a very good imitation of diamonds. With zircons, providing one is a reasonably well-versed jeweler, gemologist or probationer, it is a fairly easy matter to see quite distinct double refraction evidenced by the doubling of back facets. I have quoted jeweler, gemologist or probationer because one presumes that readers of the Journal of Gemmolgy are just that. Colorless zircons are brittle and soft and usually reveal this by the very frequent chipping and worn appearance of the facet edges. Zircons have a muzzy look when viewed through the table facet with a lens. This ‘out of focus’ appearance is due to the market double refraction.

Should very small rose-cut or brilliant-cut colorless zircons be used in cluster setting the stones may be slightly rubbed or no double refraction easily recognizable, and then the spectroscope comes into its own. The well-known absorption band seen in the red end of the spectrum at 6535 angstrom is completely diagnostic for zircon—very easy to find and practically infallible in showing (besides other lines) in zircon. The spectroscope knows no barriers of size, cut, polish, rough, or water-worn.

Synthetic rutile with its play of color should never cause any hesitation even to a non-gemologist jeweler. Its large double refraction is to startling as to make the doubling of back facets look like separate distinct facet edges. Synthetic rutile is the extrovert among stones. Rutile does not exist in nature as cuttable rough material and certainly no rough or natural rutile ever looked like a poor relation to the poorest quality in diamond used in jewelry. Synthetic rutile in any case cannot get that white or colorless aspect of diamond. It always look a little yellow or off-white.

Strontium titanate is the most dangerous to the jeweler. Although it has too much fire in its pristine state it can be dangerous when a little rubbed or dirty. Strontium titanate is very soft and has a slightly molded look if observed carefully at the facet edges with a lens. Apart from an old-fashioned (but very practical in this case) hardness test, there is little one can do to identify a strontium titanate except by examining certain abrasive marks seen under laboratory conditions. For an artificial stone it is quite expensive—the smaller sizes are more expensive per carat than the larger sizes. Its brilliance and fire cause it to stand out as superior to diamond but its soft look and rounded facet-edges betray it. Strontium titanate is much heavier than diamond and if a stone is loose this factor can be used against it. Quite recently a friend of mine ordered a strontium titanate with a 1 carat diamond spread. In actual fact it weighed 1.61 carats. So that comparison of a stone by gauge to actual scale weight can be very informative.

High refractive index pastes are sometimes deceiving. One always thinks immediately of swirl striae and bubbles, but they are not always seen. In an antique ring of backed table-cut stones it is not always wise to attempt a hardness test. It a spinel refractometer is available a refractive index reading is often possible and here information is quickly gained if the resultant reading seen on the scale of the spinel refractometer has a color fringe. These color fringe readings indicate paste as opposed to glass. Because of the high dispersion of most pastes a colored fringe or edge is seen as the reading on the refractometer scale. Pastes from 1.61 upwards especially towards 1.65, 1.66, 1.67, etc. show this effect clearly. Similarly if a paste-set article cannot be checked on a refractometer for various reasons often it will yield information if immersed in monobromonapthalene. Monobromonapthelene has a refractive index of 1.66 and pastes around this reading will tend to disappear or the facet edges fade when a stone of jewelry is immersed. In contrast, diamond will stand out clearly. Certainly it is helpful also to find swirls or bubbles, but immersion will readily distinguish paste from diamond, and it is the elimination of suspects from diamond we aim at in this article, not necessarily complete identification of the stimulant.

We have dealt with the stones most likely to be met with as diamonds and discussed their characteristics. What if the stones are diamonds? Nowadays with the decline in horse drawn traffic anvils are not so common, so the gemologist usually equips himself with a large cheap synthetic ruby or sapphire with a large table to it. Diamond will scratch synthetic ruby or sapphire. Brute strength is not required. Diamonds have a clarity and brilliance unapproached by any other stone. Very often the girdle of a diamond has a small natural unpolished facet left on it, either by design or fortuitously, and this is very helpful. Although this is not a laboratory test, I have frequently noticed how even a light touch of the finger on the table facet of a diamond leaves an imprint of grease from the skin (or fingerprint) very sharply defined indeed bearing ample witness to the well-known affinity diamond has for grease. Diamonds will stand out sharply in methylene iodide. Diamond facet-edges have a quality of finish and a degree of hardness not seen in any other stone. Simple tests to prove diamond are the hardness test against synthetic corundum and high relief in methylene iodide. Laboratory refinements, of course, are infinite, and include fluorescence, under X-ray excitation or long and short wave lamps. Electro-conductivity tests also play their part, but by and large it is: Look first, lens second, opinion third and then proof by whatever methods seems obvious, expedient and positive. The X-ray excitation or short wave lamps, etc. are refinements but all add to and play very useful part in this identification of diamond, this common stone that seldom needs testing—or does it?

A reiteration of factors in the foregoing to eliminate diamond simulants from diamond may be helpful here. Synthetic spinels are single refracting, disappear in methylene iodide, and are scratched by sapphire and diamond. Synthetic sapphires are double refractive, often have bubbles and chatter marks, and are scratched by diamond. Zircons are soft, brittle, strongly doubly refracting, have a 6536 angstrom line and are heavy stones. Synthetic rutile has tremendous double refraction, strong play of color, and is markedly off-white. Strontium titanate—tremendous fire, singly refracting, soft facet edges and girdle, ‘centipede’ outline scratch marks. High R.I pastes—sometimes bubble and swirl marks, are very soft and disappear in monobromonapthelene; heavy, color fringe on spinel refractometer; single refraction.

Diamond is single refracting and has an affinity for grease. It will easily scratch all other gemstones including sapphire and ruby (both natural and synthetic). Sharp relief is shown in methylene iodide. It has optical clarity, extreme hardness and polish, and sharp facet edges. Diamond fluoresces milky-blue when excited by X-rays. It has characteristic carbon inclusions and may show naturals on the girdle.