CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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 Post subject: Re: Walking my baby (paraiba type tourmaline) back home.
PostPosted: Mon Oct 22, 2018 7:36 pm 
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Joined: Thu May 12, 2016 2:18 am
Posts: 642
In my opinion you should have a GL Gem Spectrometer or equivalent if working in Tourmaline, and other stones where chromophore is a significant factor for value or treatment. I have gotten parcels of rough in Africa where it was majority Cuprian, but almost always some are not. The Africans sort by color, and are, shall we say, "optimists" when selling. The GL is portable enough to take to the field.

As Bruce says, light source is important. I have three light sources in addition to daylight at my cutting bench. The variety of lighting encountered today is so much more varied than it was just a few years ago. Color shift can be a much more significant thing these days.


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 Post subject: Re: Walking my baby (paraiba type tourmaline) back home.
PostPosted: Tue Oct 23, 2018 7:14 am 
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Joined: Thu Sep 25, 2008 10:33 am
Posts: 833
Location: Mars PA
I have two daughters and a son, and I wanted to give each one of them a tourmaline set in a piece of jewelry. My younger daughter turned out to be the challenge. She wanted a bright yellow tourmaline without a greenish cast. I know that canary is "new" and fills in a "missing" color in tourmaline, but that is not true. Perhaps yellow was not a "commercial" color before canary hit the market, but it was out there.

The problem with yellow in tourmaline is its color is very sensitive to differences in how individuals perceive the color and differences in lighting. In my quest to please my younger daughter I cut all the "yellow" tourmaline I could. (I also love it.) Since yellowish green is one of the most common colors in tourmaline most of the cut stones were too green to suit her. I finally found a pure yellow that was quite pale, but refused to go green. ( vivid yellow and pale yellow are in two different worlds when it comes to rarity in tourmaline in my opinion.) She was happy and I was relieved.

Now why I rambled on about this history, on this thread was a gem I cut that I came to call my "sun dial". I think people talk about natural light very easily, but it is a much more complicate light source than just going outside and looking at the beautiful gem you just finished. The time of day and even the direction you are facing when looking at the gemstone can be important. With my "sun dial" it is so sensitive to how much yellow the natural light has and how it changes with the hour of the day, that I can practically keep time with it. The changes in color with different lights is certainly not unique to yellow green. The whole subject is covered under metamerism (My spell checker does not have the word and I hope I spelled it correctly.) and it is a critically factor in giving man the ability to see colors.


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