January 24 Through February 4—TUCSON, ARIZONA: Annual show
Welcome to the GemologyOnline.com Forum
A non-profit Forum for the exchange of gemological ideas
It is currently Thu Mar 28, 2024 6:14 am

All times are UTC - 4 hours




Post new topic Reply to topic  [ 1 post ] 
Author Message
 Post subject: Surprises looking at tourmaline with a spectrometer.
PostPosted: Thu Feb 14, 2019 12:24 am 
Offline
Gemology Online Veteran

Joined: Thu Sep 25, 2008 10:33 am
Posts: 840
Location: Mars PA
Nothing too new here about color in tourmaline, but I would like to share some undercover work done with my spectrometer. The four gems under the spotlight today are:

1, A paraiba like blue tourmaline. It is very much like sea form, but both axis are blue rather than a yellower C axis and a bluer A/B axis in sea foam. Its tone value moderately light and is not very dichroic. In the old days I would call it a very nice, small, bright indicolite.

2, A moderately bright greenish yellow tourmaline that is coming out of Tanzania at this time, I think. Its saturation level is low enough that it really does not jump out at you.

3, A deeper toned tourmaline with probably a bit more brown in it than number 2 above. I bought it as a usambara effect stone and maybe it is. I only have one sample of usambara that is light enough to measure with my spectrometer. This stone is certainly lighter than any uambara I have ever seen and the combination of both stones does not have enough reaction depth to show the color flip.

4, A nice moderately flawed minty blue green tourmaline. It is one of four stone I cut from a big hunk that I bought as cab grade. And the rough did have some major problems and associated yield problems. Still I love its color cutting the suite and spent a lot of time .

I set up the spectrometer which is easy to do and looked at number two. As I said, I thought it was from Tanzania and probably a Dravite, the species. Tanzania has many kinds of gems including chrome tourmaline, but has very few pigmatites suitable for Elba I looked at ite. It is easy to see that it is an Elbaite with my spectrometer. (Chrome tourmaline is easy to determine whether it is an Elbaite and not Chrome or a Dravite, too.)

Then I looked at the darker toned greenish yellow hued tourmaline. It sat right up and announced it was a Dravite. Looking at both stones, that I thought came from Tanzania, I would have guessed that they were both Dravite, the species. So much for the eye.

Then I looked at the paraiba like tourmaline. Its absorption curve was from pure iron with all the absorption being in the red end of the spectrum. I was surprised to see that the iron peaks in the greenish yellow Elbaite and this stone were almost identical. So what had taken a blue and made it into a much more common and less desirable greenish yellow? Titanium or some combination containing Manganese was the culprit. Iron showed up as the potential star counter what I use to think in my youth..

I was not completely surprised that the minty blue green tourmaline was colored by copper, only because of where it came from, Mozambique. Since copper as a chromophore in tourmaline is really rare and iron can copy its color in the blue green range, I would never declare this stone contained copper without the spectrometer. With stones in the blue green range of colors, it is an easy and consistent determination. Making it was why I bought the spectrometer in the first place.

As you can see there is hidden world of wonder in tourmaline that a spectrometer help you enter.

Bruce


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 1 post ] 

All times are UTC - 4 hours


Who is online

Users browsing this forum: No registered users and 31 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
Gemology Style ported to phpBB3 by Christian Bullock