Grain Boundary Diffusion Part III

[YourGemologist]

To receive the whole story from our Tucson Seminar please use these links: http://www.schoolofgemology.com/Diffusion1/SectionOne.pdf

http://www.schoolofgemology.com/Diffusion2/SectionTwo.html

27 January 2009

The Grain Boundary Diffusion: Understanding the Process!
Part III of III that will lead up to the ISG at Tucson 2009 Seminar!

In Part III of our report we are going to change course just a bit due to a lot of inquiries from many of you regarding the term: Grain Boundary Diffusion of Transition Metals. That was a big change to throw at you last week as we continue to fine tune our investigation to more appropriately reflect the nature of the treatment.

Semantics and Terminology

Before we start we need to address one issue: semantics. It has become rather astounding to us grass roots folks just how bitter the battle is between scientists regarding who gets to “name the dog”, as it were, in this case the diffusion of transition metals into gemstones. We have experienced scientist pitted against scientist, not because of the accuracy of the ISG reports but rather for whose technical name of the process is the correct name to use. I want to let everyone know that we don’t care who gets to name that dog. What we care about is that the treatment is identified, verified, and properly disclosed to consumers. For us, we go with the one whom we trust the most, Ted Themelis. And according to Ted the name is Grain Boundary and Layer Diffusion. Case closed, dog named, back to the diffusion explanation.

Grain Boundary Diffusion

Rather than go into a lot of in depth scientific detail that will bore me as much as you, let’s instead go to some graphics we have prepared that will hopefully allow greater understanding for grass roots folks out there.

No gemstone crystal grows as a perfect crystal. They all have minor twists and pushes, misalignments and structural defects. This creates tiny voids within the crystal structure that creates conduits by which foreign material, i.e.….transition metals, can enter and become an additional coloring element to the gemstone. This can happen naturally over millions of years, or with a little help from a Thailand gemstone cooker, it can occur artificially over a period of hours, days or months. The process can be done using high heat and/or pressure, and can be assisted by irradiation of the stone to further open existing grain boundaries, or create new ones by use of irradiation knocking what are known as “nanotubes” into the crystal structure. The main issue is that by all accounts, this is the method being used to artificially diffuse metals into gemstones to create colors.

Here is a graphic description of the concept:




At left is the basic shape of the tourmaline crystal structure. This is the shape that each single building block of tourmaline crystal will take when all of the various elements all are bound together.



The tourmaline crystal will grow as each block joins together with other crystal blocks to create the tourmaline gemstone crystal.










However, the crystals do not grow in perfect symmetrical shape. At times there are small distortions that occur that leave open tiny gaps that create tubes into the interior of the crystal structure.











The red areas in the graphic at right are the grain boundary openings in this tourmaline crystal drawing.












When this happens in the three dimensional crystal structure you have a planar grain boundary opening that extends as a tube into the crystal by which atoms can enter.









Sometimes these grain boundaries are on the atomic level size; sometimes they can be large enough to be seen with a microscope or even with the human eye. But the important issue is that with the proper conditions created, transitional metals such as chromium, iron, manganese and others can be forced into the gemstone through these grain boundary tubes as seen at left in a side view of the crystal structure.








As the atoms enter the crystal through the grain boundary opening they are able to disperse into the gemstone. Sometimes moving uniformly into the gemstone to create a completely new color for the gemstone, and sometimes congregating in higher concentrations in one part of the stone than other parts, thereby leaving unusual color zoning throughout the stone.










An example of this is seen at left with a 90x microphotograph of a growth tube in tourmaline that has turned into a grain boundary by which an artificial color has been introduced into this crystal. You can see the tube in the center of the diffused red colors that surround the tube as the colors (transition metals) are diffused into the stone through these tubes. In this image the red color has not diffused into the stone very far from the tubes. Below is another example.












Another close up of this feature showing tube structures diffusing red colors out into this otherwise green tourmaline. This is a demonstration of the red and yellow matrix material we have isolated as being used to perform this diffusion process. The ISG has collected many specimens of this feature and has commissioned LA-ICP-MS and XRF testing to identify the materials inside some of these tubes. We have also completed testing on what impact this has on the elemental structure of the tourmalines.





And finally, a look through our immersion cell at an example of a tourmaline where the colors diffused in a strange pattern in the stone. The brown lines are large growth tubes that have become the grain boundary openings. The color of this tourmaline demonstrates just how much the diffusion process can create something unnatural looking for tourmaline, which is one of the methods of identification of this and all diffusion treatments. We will have more on some of the identification methods we have found at the Tucson seminar.

We will be presenting our findings in our previously announced seminar on the Grain Boundary Diffusion of Transition Metals into Gemstones at the Hotel Arizona on Friday, 6 February at 1500hrs. After the week at Tucson we will resume our reports to all of our ISG community through this newsletter and additional reports that we will make available to the industry.


We need to again stress that the diffusion of gemstones using this method has been around for quite some time, and is a viable treatment that will serve to make a greater number of gemstones available to the market for years to come. But we must have proper disclosure of the treatment or else it could greatly damage consumer confidence in the industry.

It is for the purpose of proper disclosure and protecting consumer confidence in this industry that we go forward with this research and the reports.

We look forward to seeing you all in Tucson!

Robert James FGA, GG
President, International School of Gemology



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©2009 International School of Gemology . ALL RIGHTS RESERVED

All images are taken using the ISG Student Reference Collection of gemstones in the ISG office. We do urge and support sharing of this information in its entirety, with copyright notices intact, to others who are interested in the study of gemology. Jeweler’s Associations are welcome to distribute to your members.



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This new type of diffusion is truly a remarkable discovery in the field of gem treatments, and Robert you deserve a huge reward for this outstanding achievement, looking forward to your upcoming book