Cuprian tourmaline and the spectrometer

I have certainly written about my spectrometer and the testing I did to discover which tourmaline in my collection contain copper, but not in this focused a manner before.  I decided to write this discussion because there was a debate about one of my gemstones and its cuprian content.  It ended up being sent to the GIA and they determined that it did NOT contain copper.  This was what my spectrometer had indicated.  You certainly could not make the copper determination by looking at color alone.  Its bright medium pastel hue, medium tone level and clean body are exceptional, without regard to copper, but its “neon” level is more in line with sea foam tourmaline from Afghanistan than cuprian tourmaline from Mozambique.  (The gemstones clarity was actually a factor against it being cuprian.)

The success rate of the spectrometer, in the range of color associated with Paraiba/paraiba like/paraiba type tourmaline, with identifying copper is excellent.   The only color range that gives ambiguous results with my spectrometer is the reds, because the spectrometer’s range does not included enough of the infrared.  It turns out that in some red tourmaline (or tourmaline with a significant red factor in its color) the manganese plus three ion has an absorption peak in the near infrared that is similar to copper.  Now combine that with a peak from iron, that is similar to first copper absorption peak in the red end of the spectrum and it can lead to uncertainty about the red containing copper.

A spectrometer with a wider range in the infrared could easily resolve the issue, but I do not have access to one at the moment.  I hope to make such a determination soon.  It is interesting to me that many, if not all the reds chosen by my spectrometer to have indications of copper, are exceptional bright gemstones.  (Some do not have great crystal, which could inhibit their brightness.)  Whatever comes out for the individual gemstones (a couple of large rich pink to purple tourmalines have been tested and had their copper content confirmed) I would really like to know why absorption by the manganese three ion on the edge of the visible range of light could possibly produce brighter colors.

It is fortunate that with the advent of modern solid state spectrometers, without mirrors, thousands of tourmaline gemstones can be screened for copper quickly and easily.  Unfortunately the eye can not perform the service in lower grades of Paraiba/paraiba like/paraiba type tourmaline, because there hues are not unique and they do not display a high enough level of “neon” glow to be distinctive.

Bruce

 

About Bruce Fry

I was born in Summit, NJ in 1947 and graduated from Summit High School in 1966. I graduated from the Colorado School of Mines in 1970 and after spending another year in graduate school, I left to see the world of Brazil. After spending some more time discovering myself, I ended up working for the Commonwealth of Pennsylvania for 32 years as an Air Quality Engineer in the Department of Environmental Protection. I retired in 2007 and took up faceting gemstones again after a long hiatus that reached back to my twenties. I had started cutting cabochons when I was 13 and bought my first faceting machine when I was 15, but ran out of money and time until I retired. My great love in gemology is tourmaline and the collection presented here represents my effort to get as much beauty and variety in the colors of tourmaline as I can. I was particularly lucky in being able to get unheated cuprian tourmaline before copper was discovered in gem grade tourmaline from Mozambique.
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