Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
1bwana1 wrote:
Tourmaline that has Cu is getting a premium price even if the color is ugly. Makes no sense to me. The reason Cu stones were originally desirable was that the Cu produce exception neon colors. An ugly stone is an ugly stone no matter the trace element in my opinion.
Burmese Ruby, Kashmir Sapphire, come to mind. Often the descriptive adjective trumps the quality of a gem.
"I believe that G.I.A. uses the GL-Gem Spectroscope, as do a number of other of the big labs. I have one of these in my lab and it works great."
I would be very surprised if they did. When I worked there in research we used a Hitachi 4001 UV-V-NIR spectrophotometer. I also ran many cuprian tourmalines on that machine. Other divisions had various versions of UV-V-NIR spectrophotometers. However, I did a lot of comparison testing between the $50K 4001 and Ocean Optics spectrometers back in early 2000 and in some cases they provided better data and were definitely faster.
“The VIS-NIR digital spectroscopes that the labs like GIA use to make the call are only qualitative, not quantitative.”
It is interesting to note that the qualitative method that has sometimes been used in the past can lead to issues. A IIa diamond was defined by the absence of nitrogen in the FTIR spectrum. When I joined research, we used a Nicolet SX-60 FTIR but after a while we upgraded it to a Nicolet Magna 560 FTIR. The better sensitivity of the new instrument effectively changed the bar for defining a IIa diamond.
Thanks for posting that chart. i would definitely make the call as non cuprian based on that chart.
I have some stones that have a broad low hump at around the 900nm line. Usually I expect a much sharper peak there to make the Cuprian call. I am really not sure about these, so i was interested to see your chart. But, not the same at all.
Joined: Thu Sep 25, 2008 10:33 am Posts: 840 Location: Mars PA
Hello again,
After being lost in developing my new website, I decided to check on something else I might help with.
First I would confirm that I have never read of a study to equate copper concentration and if the gem could be called Paraiba/cuprian etc.
Spectrographic work is done to determine that copper is a chromophore and therefore cuprian, because in part surface contamination can deceive some quantitative tests. (simply rubbing the gem's surface with copper metal does the job) and they could be used to get a quantitative number for a gem, but you would have to know the light's path through the gemstone which would not be easy, along with data to calibrate it.
Laser ablation is the way to go if you want to understand the complex world of tourmaline (Like the cuprian reverse Alexandrite color changer I call Laurellite.) It still takes a high-quality effort to get good numbers.
The numbers in the original Lab report in G & G about the color changer from Mozambique (public discovery of copper in tourmaline in a Mozambiquan gem) that I sent into the GIA were very inaccurate. I know this because the same gems have more than one set of little pits on their girdles. (Laser ablation)
The graph presented on this post is NOT cuprian.
A final point that should be thought about when you want to discuss tourmaline and color.
Why is there a category/variety of tourmaline called "paraiba like"? Because iron can duplicate the color of copper in tourmaline period. Without the glow-like quality of Paraiba, color alone does not make it unique. (unheated purples may be unique, I am still waiting for an iron example.) Look at top-quality indicolite from Namibia and Pakistan/Afghanistan. Research done by the GIA on fluorescence in diamonds could help explain the glow-like quality of some tourmaline. The GIA found that defects in the diamonds, rather than fluorescence, are the principal cause of reduction in contrast (milkiness) in diamonds. It is known that at too high a concentration copper exosorbs into native metal inclusions. I don't think that it is too much of a stretch, to visualize copper causing defects in tourmaline that are too small to be resolved with a light microscope when copper is in the right range of concentrations, but they make the tourmaline less transparent.
I have pushed for research on tourmaline for too many years. New powerful tools have come along, but the greatly increased value of the material to test limits the projects that are reasonable. I think that many questions about color and tourmaline will have to wait until they can easily be synthesized.
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