Post subject: Proof that heating of pink tourmalines IS done
Posted: Mon Oct 26, 2009 10:37 am
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Joined: Thu Apr 26, 2007 11:03 am Posts: 191
Most posters here talk about pink tourmaline being heated as though it is widely known but is there any proof? If it is so difficult to detect, then how is it detected in the first place? Are there any scientific papers that discusses the heating of pink tourmalines?
Good question. Most tourmaline is heated to maybe 600C to drive off the iron. Now I have been told that corundum heated to that temperature, the so-called "low-temp heat" treatment can be detected but I have never heard of a method of detection for tourmaline. Pink is also irradiated but I don't know of any means of detection.
It would seem that inclusion study would work, maybe tourmaline is too low priced to have attracted interest.
Joined: Thu Dec 13, 2007 3:42 pm Posts: 4091 Location: the Netherlands
Quote:
It would seem that inclusion study would work
I doubt it. Tourmaline is lacking the tell-tale inclusions corundum has: ex/dissolved rutile, (melted) crystal inclusions etc... I've been under the impression that tourmaline needs to be very clean to be a heating candidate. Maybe that spectral analysis can do anything?
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Years ago, when I was a student in Germany, we noticed that some tourmaline had a bit of a ghost shadow when were taking it's RI.....is that making sense?
Almost like a double line.
The professor said, it was a result of the tourm being heat treated.
I have never heard that subsequently.
Joined: Wed Feb 25, 2009 5:53 pm Posts: 175 Location: Toronto
I got a huge pile of back-issues from the Canadian Gemmological Association's Journal (and FGA and GIAs), and remember something about treatment being a proposed explanation for the ghost-line on tourmalines.
Let me see if I can dig it out... but he didn't just make up the idea Barbara.
Joined: Sun Oct 07, 2007 2:04 pm Posts: 623 Location: Southern OK/North TX
The 'ghost tourmaline' thing is called the Kerez effect. There's a thread somewhere where Doos and I were discussing this. I've been able to get as many as 4 lines to appear on some of them. Here's an example from one of my stones:
I have others but they're not uploaded to photobucket yet...will dig them out. This is VERY interesting to me - I have 7 tourmalines that I can plop that double/triple line down on that scale any time I want so it's not a fluke.
NOW - with that said, I figured out what some of mine are but I'm not sure why it's happening. Notice the slant of the middle line - it actually coordinates with the angle of one of the facets in the stone. Some told me it was because I used too much RI fluid BUT I've done many hundreds of stones without this occurring INCLUDING right before and after and use the same each time.
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Perhaps, perhaps not, Squeaky.
The Kerez effect was first noted by Professor Bank at DGemG but the cause has remained speculative. It was noted in green tourmaline.
I am talking about a ghosting visible in red and pink tourmaline in which the lines were very much closer together. Perhaps this has something to do with the thermal shock created by the polishing process, and perhaps it does not.
My professor, Herr Mertens, believed it had to do with heating at low temperatures.
Again, perhaps he was right and perhaps not.
But, your picture is exceptional!!!
You should add it to the GP!
hint hint
Joined: Sun Oct 07, 2007 2:04 pm Posts: 623 Location: Southern OK/North TX
Barb - you're right, I stuck my foot in on another conversation as these aren't pink or red nor do I know if they're heated. I don't know if those you're talking about show the same thing as this Kerez effect. How do I add that pic to the GP? I'll gladly do it....
Bill,
These are mostly pavilion readings. That's how I figured out that I was seeing a reading from the facet lines on some of them. I took out the ones that were always only facet lines that I could get rid of by moving the stone a little but not enough to change that appropriate reading or removing a little liquid with paper towel tip. This left a few that give a ghost reading that doesn't disappear until the main reading also leaves.
I sure hope that made sense....these REALLY fascinate me....
Joined: Thu Sep 25, 2008 10:33 am Posts: 840 Location: Mars PA
Heating tourmaline effects the oxidation state of the transitional elements impurities, but does not effect it's bulk chemical composition. Heating also allows the elimination of color centers caused by dislocated electrons and the holes that are formed in the electronic matrix of tourmaline.
The heating of tourmaline is problematical and most tourmaline that is heated is either completely faceted before heating or at least had it's surface ground to eliminate surface flaws. This still leaves the possibility that two pieces of tourmaline that look to have identical inclusions/flaws can heat very differently. One safely passing the test with it's integrity intact and the other one destroyed. It does appear with cuprian tourmaline from Mozambique, that most of it that is heated forms at least micro fissures. Tourmaline can not tolerate the high temperatures used to process corundum or some feldspars and that makes it highly unlikely the re-adsorption of inclusions or diffusion of elements into its matrix is practical.
I have read that the differences in IR readings are caused by heating from polishing. It would be interesting to take one of the gems that demonstrates this effect and recut it using a polishing process that keeps heat to a minimum. Perhaps the IR effect would be eliminated.
Radiation with or without heating can darken pink and red tourmaline. It's effect is not limited to the oxidation of manganese. One effect is the production of a color center that can make tourmaline yellow. This effect can be eliminated by low temperature heating, but is stable under normal ambient temperatures. Other transient color centers maybe formed, but the pinks available on the market have been determined to be stable.
I don't believe that the irradiation of tourmaline by man can be detected. Most tourmaline has been irradiated naturally by radioactive potassium that occurs in the feldspars associated with pegmatites where the tourmaline was formed. Irradiation can damage tourmaline if it is not properly done in the laboratory.
What percentage of pink tourmaline is heated depends on many factors I am sure. One of which is how common an undesirable over tone such as brown is found in the pink tourmaline. I just saw a comparison of two pink tourmaline from a Brazilian deposit on the internet. The unheated, brownish gem was the norm for the deposit and the clean pink heated gem was not richer in color, but had had its brownish overtone eliminated. Both gems looked clean.
Joined: Sun Aug 30, 2009 7:55 pm Posts: 152 Location: Marion, IL, USA
In Minas, it is well known that pink tourmalines from Araçuaí are heated to try to eliminate most of the brownish undertone. Generally, there is still a faint tone of brown. Many pinks from Araçuaí are incredibly clean. I had a couple that were clean at 60x.
It is not impossible to heat tourmalines having tiny fractures.
Many rubellites that come from Africa to Brazil are cut, then irradiated, then heated.
The above-mentioned photo of unheated and heated pinks from Araçuaí is here:
Joined: Thu Sep 25, 2008 10:33 am Posts: 840 Location: Mars PA
Hi Gringa,
Does brown usually heat out of pink tourmaline from most locations? Is the heating after the irradiation done to remove a brown mask on the color?
Did you find out anything interesting about green and Paraiba tourmaline when you went to Minaes? Do you think that most pink tourmaline is irradiated in Brazil?
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