ssst. pssst. Want a hot tip? Hit the library and dredge up a gem treatment, preferably one developed years ago, so that most have forgotten it. Spice it up a bit, give it a jazzier name. Then throw a piece into a parcel of naturals and take it round to a lab on… say… New Year's Eve. Did the lab muff the identification? Great! Call in the trade media to roll out the product (and don't forget to show them the natural report). Now promote the bejesus out of it by telling people that it's virtually unidentifiable.
Sound crazy? It shouldn't. It should sound familiar, like deep surface diffusion.
Most in the gem trade have now heard of the deep surface diffusion-treated sapphires. But few know how to spot them. And fewer still realize that surface diffusion is nothing new. But good treatments are a bit like vampires. No matter how often you kill them they always come back to haunt you.
Surface diffusion 101
The surface diffusion treatment was first developed at Union Carbide's Linde division in the US. Linde, the first company to synthesize star corundum, had problems obtaining both uniform color and uniform silk in the same stone. Here's the recipe for cooking sapphires the Linde way:
Pack stones in a crucible filled with the kind of chemicals which produce both rutile silk (Ti) and color (Ti, Fe, Cr, etc.).
Heat to near the melting point (1800–1900° C) for several days or even weeks. Periodically recharge the crucible with chemicals.
After cooling, lightly repolish, and voila – the gems now have color and/or stars.
How does it work? Like this: when a stone is heated to near the melting point, the crystal lattice is expanded to the maximum. Heat it too much and the bonds break completely. That's melting (and that's tough titty if it's your stone). But if you heat it to just below the melting point, where the bonds stretch but do not break, the gem will absorb the chemicals, creating color, asterism or both. However, atoms of Fe, Ti and Cr are fairly large and so cannot move easily into the stone. Thus the color and asterism are confined to a thin layer (0.10 to 0.50 mm) near the surface. This process was patented by Union Carbide in 1975–1977 and is termed surface diffusion.
Surface diffusion works best on stones which have little or no color of their own, as it makes little sense to add color to something that already has plenty. The starting material is generally near-colorless sapphire from Sri Lanka. Due to the shallow penetration of surface-diffused color, stones must be cut (but facets left unpolished). After treatment the surfaces will be pockmarked from the high temperatures. And so the stones are lightly repolished (with the emphasis on lightly), to leave intact as much of the color layer as possible. Too heavy a hand on the polishing wheel results in both loss of color and $$$$.
Both faceted and star sapphires of blue and orange colors treated in this way first appeared in the gem trade in the late 1970s and early 1980s. Initially the stones were produced by the Swiss company Golay Buchel, which bought the process from Union Carbide. Later, stones treated in Thailand and elsewhere appeared. But they soon disappeared when gemologists became familiar with their characteristics. And by the mid 1980s they were rare indeed.
Return of the diffusion zombie
In mid-1988, I again began to see surface-diffusion treated corundums, or SDTCs, in Bangkok, but this time with a more sinister glint. It had always been in the back of my mind that, rather than taking near-colorless sapphires and treating them to a deep blue, some day treaters were going to take a stone which already had a lot of color of its own and just give it, via surface diffusion, a bit more. This is what happened in mid-1988 – to such an extent that I reported the matter to the International Colored Stone Association.
Treaters were taking medium-quality Kanchanaburi sapphires that had a lot of color of their own, but were heavily zoned, and diffusing their problems away.
Because certain burners were disappointed that their best efforts could still be identified, they embarked upon a campaign of threat and innuendo against Bangkok gemologists. But this had little effect, eventually giving way to…
The deep diffusion mutant– Kill it before it mates
Beginning in mid-1989, I heard about deep surface diffusion treated corundums, or D-SDTCs. Yes, that's right – deeeeep surface diffusion-treated blues. In April, 1990, after learning who was involved, I approached the principals (Las Vegas-based Gem Source) for more info. They were generous in offering both samples for study and information about their marketing plans. They planned, and have, sold the material for what it is – surface-diffusion treated corundum.
But all was not well in the Land of Blue. Jeffery Bergman of Gem Source told me in June 1991 that he had received more than ten death threats, presumably coming from paranoid Bangkok dealers who feared that the D-SDTCs would wreak havoc on the natural sapphire market. Bad craziness indeed, but it couldn't help but bring a smile to my face – the very idea that something might destroy the natural blue sapphire market. "What market?" thought I, "there are practically no natural blue sapphires being traded in 1991."
Deep diffusion– total oneness (or just TV evangelism)?
Do deep surface-diffusion treated sapphires (DSDTCs) represent gem treatment nirvana? Or are they just the Disneyland representation of the place? The original Linde patents claimed a maximum penetration of 0.50 mm, while the deepest penetration measured on the D-SDTCs is 0.40 mm. In all other respects they resemble the Linde material. So, gentle readers, this is what I say: "No way, Ray!"
But this does beg the question of why deep diffusion has created such a furor in Bangkok when the earlier activities by Thai burners raised not even a ripple. Good question. Mighty good question. It just may have something to do with the fact that the profit from the earlier doings went to local traders, while those of today go to heathens from out of town. Or maybe (and hopefully) people have finally realized that the gem-treatment genie, once out of the bottle, is a bit difficult to contain.
Let's change the subject. To stamps. Yes indeed, the little sticky things one affixes to envelopes. Daydream for just a moment…
…Here's your nightmare. A postage stamp was produced in Burundi in 1945 and is considered the world's most valuable collectors' stamp, primarily because of a printing error. It seems that the dot on the "i" in Burundi was forgotten in one batch. And before the error was caught, a few went out. So this is the world's rarest postage stamp. Now you, being the clever type that you are, get hold of some of the regular stamps and carefully erase the dot on the i. Then you sell them as the very rare type. But horror of horrors, your customers call in the police. As the police take you away shrieking and babbling you protest: "Why me? I didn't add anything. Furs are treated, fruit is treated, everything is treated nowadays." But the cops do not listen. Instead you are thrown in some hell hole of a jail with inmates whose only retort is "Why you? Why you ? Ha ha ha ha hee hee ha ha ha……… bend over."
Distracted to distraction
Today it is with sickening regularity that when a new gem treatment is introduced to the trade, attention is immediately focussed on its stability, rather than on the fact that the stone is treated. "Boys will be boys, dear" is the effective refrain. And while the police argue over whether the "boys" shot her with a gun or stabbed her with a knife the victim bleeds to death. Of course, we do have to play our part in this little charade, don't we? Because the fact of the matter is that precious few of the precious stones we trade today started out that way. I'm Chicken Little. And yes, the sky is falling. It's coming down all around me.
About the author
This article was first published in JewelSiam (1992, No. 3, May–June, pp. 83–86). For more on this subject, see also:
page is <http://www.ruby-sapphire.com/vampire_blues.htm> v.