a publication of the International School of Gemology 26 September 2013

A Most Unusual Tsavorite Garnet

The artificial color infusion of garnets dates back to the earliest patents issued for the color enhancement of gemstones. Today we have verified specimens of color infused garnets of a variety of types including the one you see at left that was treated by the same basic method as the previously demonstrated color infused tourmalines and claimed "Tibet andesine". The incomplete color penetration and zoning is also seen in color enhancement treatments such as the beryllium diffused sapphires that astonished the market over a decade ago.
Over the years efforts have been made by various gemstone "cookers" to treat light brownish grossular garnets with chromium and other elements to create the more valuable green grossular garnet known as tsavorite garnet. Below is an example from the ISG Study Collection showing the incomplete color infusion of this brown grossular that failed to fully take the color enhancement.
In the images below one can see two features that have become ubiquitous for this type of color infusion: the fused quartz crystals from the treatment slurry that gets wedged in surface breaking fissures (below left) and the unnatural color zoning (below right).

The ISG office recently obtained the gemstone seen at left that was sold to us as a Tsavorite garnet by a dealer in Desoto, Texas. The package arrived wrapped in a plastic bag containing easily identifiable Thai writing and numbering. The eBay dealer we purchased from had been removed from eBay for deceptive selling by the time our package arrived so we could not contact them for more information. The Tsavorite garnet that we received (seen at left) is obviously a Most Unusual Tsavorite Garnet.

We first tested the gemstone to insure that it was indeed a grossular garnet. Our Enwave Raman Microscope, ISG refractometer, ISG polariscope and Gem-A Chelsea Filter quickly confirmed that this was indeed a grossular garnet with a green color caused by chromium. Not enough Cr to show clear lines on our MDM UV-VIS-NIR Spectrometer, but sufficient to show a noticeable red color in the Chelsea filter.

The next thing we noticed is obviously that the color of the garnet is in splotches and not uniform, as you can see below. As we always do in a case like this we turned to the experts, in this case to John Koivula and the PHOTOATLAS of Gemstone Inclusions, Vol 2, page 469 that shows a somewhat similar inclusion in a grossular garnet from Quebec, Canada. However, the pattern of our inclusion is significantly different than that shown in the example of the PHOTOATLAS, and as Mr. Koivula noted in his text: "Such gems are rarely encountered and are generally considered collector garnets".

Given this caveat on the potential of our having obtained one of these rare "collector's garnets" from an eBay dealer, we decided to investigate further rather than declare ourselves to be significantly financially enhanced by our serendipitous find.
When we placed the stone in our infamous Dixie Cup Immersion Cell, the true formation of the green color splotches because apparent as seen below. This is where it got interesting...
When we viewed the green color splotches through our Meiji Techno GEMT-4BFDF microscope it quickly became apparent that the colored areas were structures that are classified as "treacle" by Robert Webster (GEMS, 5th Edition page 201) that appear in the brown to orange hessonite garnet. And if one views the image below of one of these colored areas showing the "treacle" formation, the comparison quickly becomes obvious, as does the question of how does brownish orange hessonite garnet crystal structure get into a green tsavorite garnet crystal? Is it possible that a lightly colored hessonite garnet had been color infused to create a tsavorite garnet? Since they are both grossular, the infusion of a certain amount of chromium into a lightly colored hessonite garnet would turn the far less valuable hessonite into a far more valuable tsavorite with little potential for separation due to both being grossular garnets. Is that possible? Absolutely. But there was more...
After several years of research and litigation over the Tibet andesine issue, we have established guidelines regarding features of gemstones that have been color infused. One of these is surface etching of the gemstone during a treatment process. Below right you see an image taken in the ISG office in 2010 of the surface etching and fused quartz micro-crystals on a facet of a Tibet andesine that had been subjected to color enhancement by copper infusion. Below left is a very similar appearing feature in the surface of our Tsavorite garnet. By Raman we confirmed that the micro-crystals are quartz and are virtually identical in size and shape to those found on the Tibet andesine.
Perhaps more important is what appears to be some type of residual flux that is common to both our Tsavorite garnet and the Tibet andesine. As seen below, the residual material of this type was first seen as residual flux in Themelis' book on the subject, and we found it in our Tibet andesine study as seen below right, and now we found it again in our new Tsavorite garnet as seen below left. By Raman the material is virtually identical, although we do not know the exact identification without advanced testing.

Conclusion: There are a number of issues that prevent a diagnostic conclusion on this one.

  • The PHOTOATLAS is the only reference we can find on those naturally occurring green splotch formations in the "collectors" type garnet from Canada. As a result we cannot further confirm that this might be the explanation.
  • The surface etching and residual flux-like material on the Tsavorite garnet that closely matches the etching and material on the diffused Tibet andesine is clearly circumstantial evidence but again, is not diagnostic.
  • We do know that garnets are being color enhanced by diffusion since we have a specimen from the labs of Ted Themelis where he has successfully done the treatment.
  • We also know that the "treacle" formation of this garnet is well documented to be in hessonite garnet, but hessonite is not reported in any text we could find to be colorless with bright green splotches and react to a Chelsea filter for chromium content. We further find no report of Tsavorite garnet having treacle structure. Also, the stone shows no inclusions typical of Tsavorite garnet.

Based on the many reference books and periodicals that we consulted regarding this stone, it does not exist. And yet....there it is, a very unusual Tsavorite garnet. Is it the product of some Thai cooker trying to enhance a low quality hessonite to a higher value Tsavorite? We cannot answer that with what we have at this time. But one thing is clear, when the text books eliminate all possible answers, and all of the test results sum up to a big zero for possible candidates, we have to refer to Occam's Razor that states (in our own vernacular)....

Given a whole bunch of possible answers, the simplest of the answers is most likely the correct answer.

The possible answers are:

  1. We have obtained from a dealer (thrown off of eBay for deceptive selling), a very rare and valuable collector's quality garnet from Canada, or
  2. We have a Tsavorite garnet that happens to have a "treacle" crystal formation of a hessonite garnet and a color pattern not documented in any known gemology text or reference book, or
  3. We have a poor quality hessonite garnet that someone tried to treat to become a Tsavorite garnet, it failed so they sold it on eBay to see what they could get for it.

We are going to leave the answer up to you. Perhaps someone out there has seen these before and can tell us if the answer is one of the three above, or perhaps there is an answer #4 out there somewhere unknown to the experts (and us). Either way, it underscores the fact that in today's gemstone markets we are seeing strange and unusual specimens creeping up in the online selling venues, and there is never a dull moment when shopping for the unusual.

In this case, we have a Most Unusual Tsavorite Garnet. Or do we?

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