There's a Rumble in the Jungle: The Sapphire Face-lift Face-off Saga
Abstract: Although not exactly new to gemology, surface ('bulk') diffusion-treated corundums (SDTCs) have recently been creating quite a stir in world gem markets, particularly the important Bangkok corundum market. The following article examines the subject in detail, starting with the original Linde patents and continuing up to the present day "Deep Diffusion" treatment. Methods of identification are discussed, as well as much of the backroom politics and maneuvering which has surrounded this controversial treated gem material. Also included is a quick-reference table giving a detailed summary of the key identifying features of all types of natural, treated and synthetic corundums.
Funny isn't it how some things seem to come round full circle. That certainly seems to be the case with the so-called "Deep Surface Diffusion-Treatment" that's been grabbing headlines of late from Mandalay to old Bombay and right back in the U.S. of A.
For those of you new to the news, since the beginning of 1990 the Bangkok gem market has been rocketed by rumors about a totally new method of putting the spit shine to blue sapphires. And separating the bulls from the fillies ain't been easy, no siree.
First wind we got of this latest treatment "taste treat" was in the fall of 1989, when an American burner mentioned some doings in the realm of surface diffusion. In hushed tones, he spoke of certain parties who were experimenting with diffusing coloring agents into blue sapphires. Nothing new here, except the results were said to be far better than the generic "Union Carbide-cum-Linde-cum-Astrid-cum-Golay Buchel-cum-prancer-cum-dancer-cum-vixen-cum-blixen…" material which hit the streets round about 1980. Word had it that this new process allowed diffusion of color into the stone measurable in actual millimeters rather than just fractions thereof. When I asked this particular source just how he had measured the thickness of the diffused color layer, he mumbled something about "optical means" and could provide no further details on just what those optical means were. Thus, I, the eternal cynic, dismissed the story as being typical gem-market scuttlebutt dredged up from some barnacle-encrusted source who had had five drinks too many. Little did I know…
To get a handle on this subject, we have to take a journey through the past. Way way back to those days of the Linde Star.  Linde Air Products was a division of the giant American conglomerate Union Carbide (of Bophal, India fame). At the outbreak of World War II, it became clear to the US government that certain materials of strategic importance might be difficult to get if no American company was manufacturing them onshore. One such material was synthetic corundum, which had hitherto been made primarily in Europe. Thus, the US government asked certain American corporations to undertake the manufacture of materials which would come in handy if push came to shoot. Union Carbide got the nod for synthetic corundum and began manufacturing it in 1942 (Nassau, 1980). Not even the most long-toothed soothsayer could have then predicted the impact this decision would have on the future of gemology.
In the late 1940s, after the war was over, Union Carbide's Linde Air Products division continued making synthetic corundum, and it was during this period that an extremely astute employee, John Burdick, noticed in one of the boules a cloudy patch which later proved to be rutile silk. Much experimentation later, the growth of this silk could be controlled to the extent that synthetic star corundums became a commercial reality. Thus, the birth of the Linde Star, which went on sale in September of 1947 (Nassau,1980).
Now while all this is well documented (Nassau, 1980, 1984a), the manufacture of Verneuil synthetic corundums was not a completely cut-and-dried process. Problems were encountered in the consistency of color and silk, in both transparent and star corundums, particularly those of a blue color. Linde toiled on. The result was a series of patents (Carr & Nisevich, 1975, 1976, 1977) providing a means of diffusing coloring agents into and just beneath the surface of corundums by heating them near to the melting point in a paste of titanium and other coloring oxides (Cr, Fe, Ni, V, etc.). Temperatures described in the patents were between 1600 and 1850° C. Below 1600°, the process was said to be too slow; above 1850°, the powder fused to the surface of the stone, requiring heavier repolishing and subsequent loss of color. The time given for the heating ranged from 2 to 200 hours, depending on the nature of the crystal.
Interestingly enough, while the patents were granted in 1975–1977, they were actually a continuation of an earlier application from 1971 (subsequently abandoned). Equally interesting was the fact that some of the later patents were issued in the name of Astrid Corporation of Hong Kong. Even more intriguing was that after the patents were granted, nothing much was heard of the process until approximately 1980.  Then it was rumored that the giant Swiss gemstone firm of Golay Buchel purchased the patent rights from Union Carbide and began processing pale-colored natural Sri Lankan sapphires by the surface diffusion process (hereafter Surface Diffusion-Treated Corundums will be abbreviated as SDTCs). This material hit the market in 1980–81, mainly in blue colors, but oranges and reds were also reported. Furthermore, it was possible to diffuse not only color under the surface, but also titania (TiO2), which upon further heat treatment turned into rutile silk, resulting in surface-diffusion star stones.
To their credit, Golay Buchel reportedly sold the material for what it was – surface diffusion-treated corundum. Unfortunately, those who bought from Golay Buchel were not always so benevolent.  One well-known Italian colored-stone dealer told the author that in Italy large quantities of this material were passed off on unsuspecting customers. He also alleged that when a local trade association was informed of this fraud, the entire matter was swept under the carpet because, in the words of my informant, "they had sold a lot of stones." Such stories are not confined to Italy; the author has personally witnessed surface diffusion-treated sapphires being sold as natural in Thailand, and the same could probably be said for any country where sapphires are traded.
Once the general process of surface diffusion became known, via cook or by book (Noyes Data Corp. published two books of gem patents which served as burners' recipe bibles), it was not long before Thailand's skilled gem chefs also had a go at it. I was shown a parcel of obvious SDTCs in the famous gem-trading market town of Chanthaburi around 1980–1981. After informing the broker that I knew exactly what he was holding (i.e., SDTCs) and telling him that I was still interested in buying a piece, I asked if the process was done abroad. The broker said, no, these were cooked in Thailand. This was made all the more believable because each stone in the lot had a far lighter color than we had come to expect from the Swiss production, no doubt because the process had yet to be perfected here. Improper treating or too heavy a hand on the polishing wheel will result in most of the color being lost.
Further evidence of surface diffusion treatment being done in Thailand was presented to the author by a close friend, Robert Stevenson, who had sent some (non-Yogo) Montana sapphires to his friendly neighborhood Bangkok burner for cooking sometime about 1980–81. Before treatment, the stones were pale green-blue. To his delight, after cooking, each had metamorphosed into deep blue beauties entirely inconsistent with what typically happened when burning this material. Elation soon turned to sorrow, though, when after cutting, the blue color had disappeared into the dust on the polisher's wheel. Specimens of this very material were donated to AIGS by this gentleman, and are pictured in Figures 1 and 2.
After the initial brouhaha caused by the appearance of this new treatment, the material generally disappeared from the market, no doubt because gemologists had become well aware of how to go about detecting it. Thus, by 1983, the AIGS lab in Bangkok was seeing only the occasional surface diffusion-treated stone. But… but…
Identifying Surface Diffusion-Treated Corundums (SDTCs)
It's Instrumental, My Dear Watson!
Identification of surface diffusion-treated corundums is a fairly straightforward process in many cases, but there are just enough potholes in the road for the unwary to fall into. In most cases, a stereoscopic microscope (with provisions for immersion in methylene iodide) is necessary.
The identification centers around the fact that, in natural corundums, crystals grow in the ground and such crystals bear only a cursory relationship to the gemstone after cutting. Color in natural single crystals forms in bands or zones parallel to crystal faces, not along polished facets, because the stone was not faceted when it was growing in the ground. Despite the irrefutable nature of that last statement, some have attempted to hoodwink people into believing otherwise. But more about that later.
Proper setup and use of the microscope is vital to the identification. The stone should be immersed in pure methylene iodide (RI = 1.735 approx.) and viewed under magnification with a diffused white (frosted) plastic or glass plate covering the light source. This is often referred to as diffused light-field illumination. Under these conditions, reflections from the corundum's external surfaces are largely eliminated. This allows the stone's interior to be seen with virtually no distortion from the reflection and refraction of light off internal and external surfaces. Thus, the true distribution of color is revealed.
Identification of surface diffusion-treated corundums (SDTCs) centers around the following characteristics, observed most easily while immersed as described above:
1. a. The color of SDTCs will be seen to be concentrated just inside the girdle of the stone, unless cutters have attempted to avoid detection by heavily repolishing the girdle itself and the facets close to the girdle. This is because the coloring agents are concentrated close to the surface; when the stone is placed table down in the immersion cell, one's line of vision crosses the greatest amount of diffused color in the vicinity of the girdle. In a non-SDTC, the color is inside the stone and is distributed throughout the entire volume; therefore, one looks through the greatest amount of color at the culet (viewed table down in the immersion cell; see Figure 3).
b. The color of SDTCs will also be seen to be concentrated on the facet junctions. This results from a greater penetration of the coloring agents at edges as opposed to the center of facets (Koivula, 198?), as well as the fact that edges are polished less during the mandatory repolishing that stones must undergo after treatment. According to Koivula, convection currents develop during the treatment that create a "dumping" of colorants at the edges. This is, I suppose, somewhat similar to the dumping of alluvium at a bend in a river. The result is a deeper penetration of coloring agents at edges and facet junctions, as compared with the center of facets.
2. If an SDTC has any fractures, feathers, cracks, pits, cavities, etc. which break the surface, then immersion reveals a "bleeding" of color into these breaks. Even well-healed liquid fingerprints can show this effect, but it is seen best, of course, in completely open cracks. The reason for the bleeding of color is that the cracks offer a ready means for the entry of the coloring agent, but are not touched during the repolishing process.
3. One further means of identifying SDTCs exists, and of late has become the most important. As mentioned earlier, color in natural corundums forms along specific directions representing crystal faces (or potential faces) of that mineral. In the SDTCs, the penetration of the process is so shallow (0.05 to 0.50 mm) that the stones must be preformed before the treatment is effected. To treat a rough gem would be to lose a large percentage of the color in the cutting process; thus, the treatment is done on preforms rather than rough stones.
Because of this restriction, and because of the high temperatures reached, the gems must be repolished after treatment. Inevitably some facets will be polished more than others, resulting in the color pattern of the finished stone following the facet patterns exactly. Some of the facets will show more color, some will show less, but the color pattern will follow the facet patterns exactly. This is absolutely without a doubt the most sure-gotcha-spot on positive way of identifying SDTCs that I am aware of.
Possible Errors in Identification (Caveats)
As mentioned earlier, the identification of SDTCs is not something to be approached lightly. The first requirement is to get your hands on some of those 100% sure-gotcha-spot on-positive SDTCs. More than one piece, if at all possible. Then these can be used to make side-by-side comparisons with unknown stones.
I am gonna say something now which I probably shouldn't say (and I'll probably regret it at age 65), but it needs saying. Any gemological laboratory can make mistakes in gem identification, and I do (read my lips) include AIGS and every other major gemological lab in the world in the discussion. For example, in November of 1988, the AIGS lab in Bangkok misidentified an ordinary heat-treated sapphire as being surface diffusion-treated because the gemologists who tested the stone saw a reflection on the girdle (the girdle was quite thick) and misinterpreted it as color concentration from a surface diffusion treatment.
In April of 1990, I was shown photocopies of two gem ID reports for what appeared (same measurements, weight, shape, etc.) to be the same stone, issued by a well known US-based laboratory. The first report stated that the stone was a heat-treated natural sapphire, while the second report said that the stone was a "diffusion-treated natural sapphire".
I bring up these two instances not in an attempt to embarrass the AIGS lab or the US lab, but to point out that even the most highly-qualified gemologists can (and do) make mistakes. When they occur, we should own up to them; more importantly, we must stay ever-more alert to ensure that the mistakes are kept to a minimum. These SDTCs can and are misidentified on occasion. Beware, brothers and sisters, beware. 
As this article was being written, a stone came in to the AIGS lab which illustrates quite nicely just how careful one must be in identifying SDTCs. The stone in question was an obvious flame-fusion synthetic sapphire, but in immersion showed the dark facet junctions normally associated with SDTCs, leading one of the two testers to initially believe it to be an SDTC. But later examination failed to show up the telltale conclusive sign of the SDTC: the color did not follow the facet pattern of the stone exactly.
This pseudo-SDTC appearance was probably a result of the rounded facet junctions common in Verneuil synthetics (due to rapid polishing), as well as the tendency of synthetic blue sapphires to have color wrapping around the pavilion facets, because the boule is colorless near the core. Again, beware. The job of gemologist becomes ever-more difficult with each passing day.
The Plot Thickens
In mid-1988, the AIGS lab in Bangkok again began to see SDTCs, but this time with a slightly more sinister glint. It had always been in the back of the author's mind that someday the treaters were going to wise up. Rather than just taking near-colorless sapphires and treating them to a deep blue, someday, somebody, somewheres was gonna take a stone which already had a lot of color of its own and just give it, via surface diffusion, a bit more ooomph for the dollar. This is what happened starting in mid-1988 – to such an extent that we reported the matter to the ICA (Hughes, 1988, ICA Laboratory Alert #12).
One additional monkey wrench thrown into the broth was the publication about mid-1988 of an anonymous pamphlet (in Thai), superficially claiming to tell the readers about gemstones, but serving primarily as a vehicle for attacking the AIGS lab  for their position on identifying SDTCs. The starring attraction of this pamphlet was a set of alleged correspondence between the anonymous author (who published the pamphlet under the moniker of Ploi Sahm See – literally 'Tri-Color Gem') and the chairman of the a well-known gemological organization. The chairman (if in fact it was his real correspondence that was reproduced) perhaps lacked full details of what was going on in Bangkok; he thus found himself unwittingly sitting in judgment on a matter of which he had only the word of a distant gem dealer.
In his first letter, the chairman attempted to avoid committing himself without further study and information, speculating about how if the stones were heated only on the surface then maybe and quite possibly, they could conceivably and perchance just might be something other than surface diffusion-treated after all.
Reproduced on the following two pages are the title page of and letters from this pamphlet (with the names removed to protect the innocent).
Readers can certainly make of these letters what they will, but I would like to call your attention to the chairman's second letter where he says:
The reasons for the decision made by the (name removed) laboratory that the sapphires were diffused are evident. The results are so subtle, however, that it would hardly make sense for anyone to diffuse a sapphire, when so little is added to the color. It would appear as if the results were what might be termed accidental, caused by heating that was effective only to a depth that did not penetrate very much beneath the surface. In other words, it appears as if the results of the treatment were confined to near the surface of the stone.
It is indeed unfortunate that the author of the pamphlet did not see fit to reproduce the lab reports on the stones, if in fact such reports were made. It seems that, while those who examined the stones did find evidence of surface diffusion, the amount of color added was far less than what gemologists had been used to seeing. Therefore they could not understand why someone would go to so much trouble to add so little color. What they misunderstood was that even a minor color zoning problem can dramatically lower the value of a sapphire. Rather than taking $5/ct material and diffusing it into $500/ct gems, as was previously done, the burners were becoming more clever. Now it was slightly zoned $200/ct stones that after diffusion might be worth $300–400/ct.
This failure to make a clear statement about just what type of treatment the stone(s) had been subjected to was misinterpreted by the author of the pamphlet as representing something akin to the GoodStone Burning Seal of Approval. Unfortunately, no attempt was ever made to contact AIGS to see if we might have additional information.
What readers won't be able to see from the above letters is the translation of the chairman's correspondence into Thai. To assist you, gentle readers, I have the pleasure of presenting a paraphrased and condensed version of the Thai translation, in epic verse form:
Tee, hee, hee, he agrees with me – my stones are not SDTCs – AIGS should take up growing tea – stay away from gemology.
All this and color pictures, too. It's a great little pamphlet. In any event, these stones were indeed SDTCs. And the pamphlet? At best, misguided; at worst, a patently transparent attempt by someone quite possibly involved in the production or sale of SDTCs to pressure AIGS into seeing the error of her way – at least insofar that, in the future, clean bills of health could be given to this fraudulent merchandise. 
The Plot Sickens
The anonymous pamphlet was just the first shot fired across our bow in the Battle of the Surface Diffusion Sea. Coupled with the one misidentification by AIGS on an ordinary heat-treated sapphire, rumors began circulating that AIGS had its head well into the nether regions of its backside, to such a degree that "many heat-treated stones were being misidentified by AIGS as SDTCs." This latter statement is absolutely wrong; to the best of the author's knowledge, only one SDTC stone (the one already described herein) has been misidentified by the AIGS lab in Bangkok.
The plot truly began to sicken when, in December of 1988, one erstwhile Bangkok dealer summoned senior AIGS staff to his office to berate our lab for not knowing the difference between stones heated only on the surface and surface diffusion-treated stones, adding in passing that Thai people didn't have the technical knowledge to perform the surface diffusion treatment. He, too, gave the same song and dance about such stones being given "surface heat only,"  and proceeded to thrust five stones at us. If, he said, these were really SDTCs, then we could have his head for breakfast (or whatever else we desired). 
So we took the stones back to the lab, gave them a look-see and, sure enough, every damned one was an SDTC. Being modest in such matters, particularly when immodesty could result in a very fast descent into the next world, we admitted to this gentleman that we could be in error, and offered to send the stones at our own expense to any lab in the world he thought capable. He replied that he didn't need any lab to tell him what he already knew about his own stones. So much for "head for breakfast."
Enter The Kitten
At this point we realized that something very serious was going on here, something that might well get someone, namely us, killed. So we informed the Thai Gem And Jewelry Traders' Association that we had no desire to go to heaven just yet, and were therefore halting the issuance of all reports on these stones until such time that the trade could decide what in fact they wanted to call them. 
The local association did look into the matter, but took so long in doing so that absolutely nothing got decided one way or another. And so, after much waiting, we finally told them that, due to the numerous complaints we had received from angry buyers who couldn't get their money back (on SDTC stones) without our report, we would go back to our old evil ways and issue the reports as we had been. This we did, and no more was heard of the diffusion madness, until…
The Deep Diffusion Madness
Starting in mid-1989, we began to hear about deep surface diffusion-treated corundums (hereafter referred to as D-SDTCs). At the beginning of April, the author, having heard through the grapevine who was involved, approached the principals for more info. They were more than generous in offering samples for study, as well as much information about their plans for the material. Since Mr. Peter Lumetta, whose article follows this one, explains what they are intending to do, I will not repeat it. Suffice to say that they fully intend to sell the material for what it is – surface diffusion-treated corundum.
Why has this created such a furor when the earlier goings-on made not even a ripple? Good question. Mighty good question. And might it have something to do with the fact that the earlier doings were "in house" so to speak, while the later shenanigans are being perpetrated by, god-forbid, heathens from out of town? I'll let readers make up their own minds on that one.
Surprisingly enough, amidst the rumors that the new D-SDTCs were both undetectable and absolutely aimed at destroying the local sapphire market, another question popped up. It seems that many potential buyers were bothered not so much by the fact that the color is entirely a work of fiction. No, what they wanted to know was if the color was a coating on the outside of the stone, or if the color actually extended inside the gem. If it is outside? No dice, no sale. And if it's inside? No Proble~mmmmm!
This little difficulty actually stems from a bit of a linguistic snag. It seems that there is no simple way of describing the surface diffusion process in the Thai language. And so such stones were previously known in Thai as ploy kluab see, which literally means "coated stones"). It didn't help matters any that the AIGS lab and the US lab mentioned previously did not describe the process on their gem ID reports in a clear and unambiguous way. AIGS has since altered the comments on its reports for SDTCs to read as follows:
This stone has been treated by the surface-diffusion process. Some or all of the stone's color is artificially created, and lies in a thin layer at and just below the surface. Repolishing/recutting may result in a substantial loss of color, but otherwise the color is generally stable.
It is hoped that these new comments will make clear the fact that the color is inside the stone. More importantly, the comments should make clear to the buyer that the color is both a product of human synthesis and is confined to such a shallow depth that simple repolishing may result in loss of color. While some may not be happy at the clarity of these comments, we at AIGS believe that a lab's actions should be governed by "Beesley's Law,"  which, in a nutshell, is: "Put yourself in the shoes of the buyer. What would you want to know if you were buying the stone?"
ICA Laboratory Alert No. 37
When we received ICA Laboratory Alert No. 37 from Grahame
Brown in Australia, we learned that unfortunately, SDTCs are again being described as "coated" corundums.
In truth, no such coating exists; the coloring agents actually penetrate the stone, rather
than growing a synthetic layer on the surface. 
…coated by a thin layer of surface diffused synthetic ruby; for repolishing had accidentally removed the thin coating of synthetic ruby from some facets. Low power microscope examination revealed identifying features of the diffused coating, that included:
…multitudes of small "bubbles" including the diffused
layer of synthetic ruby;
But Where Is The Motive, Dear Watson?
From the above description, I am not quite sure what is being described, but it certainly does not sound like a surface diffusion-treated corundum, unless the references to a "coating" are merely describing a region of the gem near the surface. What bothers me most about this, though, is the lack of motive. Why on earth would anyone go to all the bother of the SDTC process, which does cost money, when off-the-shelf Verneuil synthetics are available in orange colors anyway? Grahame Brown himself said, with more than just a touch of skepticism, "As this Australian tourist only paid US$35 for the complete suite of imitation padparadscha sapphire jewellery, one must wonder at the economics upon which the production of diffusion coated synthetic corundums is based… and the naïveté of some tourists." (Brown, 1990).
As the same lab alert also described blue sapphires (which also were diagnosed as being surface diffusion treated on the basis of highlighted facet junctions), it makes me wonder if we don't have another case of ordinary Verneuil synthetics being mistaken for SDTCs, like that described earlier. Since Grahame Brown is a regular contributor to this magazine, we hope he can shed further light on these odd stones.
Just What Is The Difference Between Premium And Regular?
So what's the difference? Is the so-called "deep diffusion treatment" the same old wine in a new bottle? Well, yes and no. According to the original patents, the depth of penetration that could be attained with the patented Linde process…
…generally is about 0.001 to 0.020 inch (0.0254 to 0.508 mm) in thickness after polishing. This thickness is determined primarily by the time and temperature of treatment… and to some extent by the atmosphere in the furnace. Generally in a reducing atmosphere deeper penetration is obtained in shorter time than in an oxidizing atmosphere. (Carr and Nisevich, 1975).
Kurt Nassau's Gemstone Enhancement (1984a) also mentioned penetration levels, which I think are derived from the same patents, but oddly enough gave different values. According to Nassau, Linde was able to achieve depths of color penetration "of about 0.004 to 0.01 inches (0.01 to 0.25 mm)." A conversion error between inches and millimeters seems to have sneaked in, too, for 0.004 inches actually equals 0.1016 mm, not 0.01 mm (as quoted).
From the above sources, I think it is safe to say that the Linde process was capable of producing color penetrations of somewhere between a few hundredths to as much as one half of a millimeter, after polishing.
The deepest we've measured on this new D-SDTC is 0.40 mm. But, we did cut open one small Golay Buchel stone which we had around, and found a penetration of less than 0.10 mm. All of the D-SDTCs we've sliced open have shown at least that much, so they do appear to be approaching the maximum limits claimed by Linde. But in light of the above, if the new material qualifies for the title of "Deep Diffusion-Treated," then some of the Linde material with the 0.50 mm penetration might best be called "Super Duper Deep Diffusion-Treated."
I'm going to beat the "natural vs. treated" horse a bit more.  In the article which follows this one ('The Diffusion Confusion') Mr. Lumetta refers to SDTCs as natural stones. On this theme, Peter and I do not see eye-to-eye.
I would like to take issue with his use of the term "natural" in the context of SDTCs, but at the same time I must confess a certain bias. Peter Lumetta is a good friend of mine. Thus, in respect of our friendship and for the purposes of this debate, I will avoid any and all mention of his family in general, specifically the matter of his questionable parentage and his father being a hapless rogue known far and wide for his love of wild women and hatred of anything falling under the vague rubric of work. These things I will not mention.
Ah… now… where were we? Yes, the idea that SDTCs might represent one category of natural stones because the starting material is natural corundum. I think it would be correct to say that Mr. Lumetta considers SDTCs as a subclass of natural corundum. Sorry, but I just don't buy it. While the crystal structure may have formed in nature, both the clarity and color have been "synthesized."  However, Mr. Lumetta does make an excellent point, which is that SDTCs are little different from normal heat treatment: one represents diffusion out of the stone (ordinary heat treatment); the other represents diffusion in (SDTCs). Mr. Lumetta argues that those who support the sale of ordinary heat-treated corundums as natural while refusing to allow the natural moniker for SDTCs are guilty of more than just a wee bit of hypocrisy.
While this idea may be considered heretical among dealers,  I agree totally. Both the normal heat treatment and the SDTCs are just variations on the same diffusion theme. Ordinary heating diffuses aluminum vacancies and hydrogen in and out; the surface diffusion process works in the same way, but with heavier elements (Ti, Fe, etc.). What upsets many dealers, I suppose, is the efficiency of the SDTC process. They should have thought of that before they started burning Sri Lankan geudas and selling them as natural. This failure to see into the gem treatment future will be the downfall of the natural stone business unless a clear distinction is made between natural and human products. Take a good look around. How many purely natural gems are left?
But With Ordinary Heat Treatments, Nothing Is Added, Is It?
Nothing could be further from the truth. Human intent is added, and that is most important in separating natural from synthetic. It's not what is done, but who's doing it. Other than that? Heat, oxygen, hydrogen and various other items are either added, subtracted or altered during the heating process. It should be obvious that if nothing were changed, then nothing would change.
Where I differ from Mr. Lumetta is in the label attached: he calls both the heated and the SDTC "natural," and following the past trade practice of allowing the term "natural" for heated goods, I suppose I should agree. But I can't. I call them both partial-synthesis processes and most decidedly not natural. The term "natural" as we define it at AIGS, is given on the inside cover of this magazine ('How to Read the Gemological Digest'). In a nutshell, a gem is natural if it has not been altered by humans beyond ordinary cutting and polishing (grinding only, no impregnations). Readers, what think ye?
The Bottom Line: Is It Really Better?
Is it really better? Nothing I've been shown so far has a better appearance in any way than the original early 1980s Golay Buchel material. All of the SDTC material I've seen to date has a peculiar "steely" quality to the blue which does not duplicate the appearance of top Burma-type natural sapphire. The D-SDTC material is no different. So in that way, no, it doesn't look better. It may be better for the producer, though, if in fact they can get better color penetration, because a deeper penetration makes for higher yields in cutting.
Is It Any Tougher To Identify?
Is it more difficult to identify? From what I've seen so far, absolutely not. It can be identified in exactly the same way as any other SDTC.
Just before our deadline for this issue, I discovered, in a dusty turn-of-the-century journal, a description of a treatment process which may be truly revolutionary! Rather than using large sapphires, it makes use of microscopic Enhanced Alumina Sapphire™ particles (impure pieces of sapphire, which could have become fine sapphires had they remained in the ground a bit longer). In a process which essentially finishes the job that nature started but adds nothing, the particles are dropped through a flame (heat only), where they enhance themselves naturally into fine sapphires. The finished product provides 100% color penetration through any size stone – they can be recut with absolutely no danger of color loss. Even more incredibly, the enhancement process can result in large sizes approaching 100 carats cut, just perfect for producing – you guessed it – calibrated sizes! It's a jewelry manufacturer's nirvana. Well, by golly, gosh and god, why hasn't anyone done anything with this process? They have. It's called the Verneuil process.
The author would like to thank the following people for assisting in the writing of this article. A collective numero uno on the list has to be Messrs. Tom Palmer, Jeffrey Bergman and Peter Lumetta, who went far beyond the call of duty in donating numerous specimens for study, in opening and copying their extensive files for the author, and for a general willingness to discuss the subject in an open and open-minded fashion. It was a pleasure to work with them on this, and I wish them the best of luck in their future endeavors.
Thanks are also due to Robert Stevenson for the kind donation of untreated and surface diffusion-treated (non-Yogo Gulch) Montana sapphires, R. Scott Montgomery for polishing some of the specimens, and to Willie Sersen and Hank Ho for their usual (and always necessary) meticulous reading of the manuscript. Finally, to Olivier Galibert, for the title photograph.
The Summer 1990 issue of Gems & Gemology magazine contains an extensive article on the deep diffusion treatment (Kane et al.). Among the more interesting nuggets contained therein is a measurement of the thickness of the diffused layer. The deepest measured by the article's authors was 0.40 mm, exactly the same as the deepest that we had measured. Of even greater importance was the discovery that some diffusion-treated blue sapphires show traces of cobalt in addition to the expected iron and titanium. This
Overall, the Gems & Gemology article represents by far the most complete discussion of the subject. It should be required reading for anyone who is interested in learning more about surface diffusion treatments.
About the author
This article was first published in Gemological Digest (1991, Vol. 3, No. 2, pp. 17–31).
Linde Star was the trade name under which the first synthetic star rubies and sapphires
This page is <http://www.ruby-sapphire.com/blue_surface_diffusion.htm>
page updated 7 March, 2013