Post subject: Re: GRS report for 'untreated blue topaz'
Posted: Wed Feb 21, 2018 12:06 am
Site Admin
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Duncan Miller wrote:
Has Dr Peretti been asked for the reasons for his finding it as untreated? When was the artificial irradiation of topaz initiated?
I suspect experimentation with irradiation started in the late 70's early 80's and was widespread by the late 80's. On September 25, 1990. NRC Information Notice 90-62 determined the requirements for Import and Distribution of Neutron-Irradiated Gems.
Post subject: Re: GRS report for 'untreated blue topaz'
Posted: Wed Feb 21, 2018 3:13 pm
Platinum Member
Joined: Fri Feb 24, 2006 1:20 am Posts: 2756 Location: Southern California, U.S.A.
For what it’s worth, there are at least two mines that are famous for producing natural blue topaz (though there may be others I’m not aware of). Neither yields natural blues in the deep color saturation of the subject stone.
One is the St. Anne’s Mine in Zimbabwe/Rhodesia; the other is the Virgen da Lapa Mine in Minas Gerais, Brazil. Based on images I’ve seen of blue topaz from these mines the stones are in the sky blue to Swiss blue range at best. The mine in Zimbabwe is said to contain a very large reserve of natural blues – if anyone has the courage and means to mine it in that very dangerous country.
Post subject: Re: GRS report for 'untreated blue topaz'
Posted: Wed Feb 21, 2018 3:33 pm
Site Admin
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Agreed. Some places in the US as well Challener family members can pop in at any time. And in Russia as well. Several xl specimens on display at the Fersmen in Moscow.
I don't think any of us are questioning the existence of natural blue topaz, just the saturation in the one pictured in the GRS report.
Post subject: Re: GRS report for 'untreated blue topaz'
Posted: Wed Feb 21, 2018 3:51 pm
Platinum Member
Joined: Fri Feb 24, 2006 1:20 am Posts: 2756 Location: Southern California, U.S.A.
Barbra, I wasn't questioning the existence of natural blues. I was trying to point out that none of the recorded locations produces blues of that hue. I'm aware of the U.S. and Russian locations, but the mines mentioned -- especially the one in Zimbabwe -- have produced tonnage in the sky blue range.
Post subject: Re: GRS report for 'untreated blue topaz'
Posted: Wed Feb 21, 2018 5:28 pm
Gold Member
Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
"No indication of treatments" does not mean that the stone has been untreated. It only suggests that the lab has found no indication of the treatment possibly undergone. Which is not difficult to believe, since the detection of such treatment is so hard to achieve.
Quote:
but why would Dr. Peretti issue this report if he couldn't be sure?
Yeah, I've collected natural blues myself in Colorado back in the day. I don't think that natural blues require irradiation and subsequent heat to remove brown either--first, because irradiation doesn't produce brown in every topaz or in every area of every topaz; only some regions of some topaz are susceptible, with some zones turning darker brown than others and some having different stability than others. second, because a number of locales produce nice blue/brown and blue/yellow bicolor stones.
As a side-note I have found a few publications on the thermolumisesence thing.
"Luminescence studies in colour centres produced in natural topaz" by Marques et al, 2000 in the Journal of Luminescence:
These authors took a piece of Brazilian colorless topaz, cut it in three pieces and gamma irradiated one and neutron irradiated another, leaving the third in its original state. Unsurprisingly they could distinguish between these three based on thermoluminescence. However there are aspects of it that puzzle me, particularly them saying they annealed the irradiated stones at 1200 c(!) and that even after that the neutron-irradiated sample was green(!). 1200 seems very high to me, much higher than you'd need to remove brown and yellow tones, which further raises the question of how the neutron-irradiated stone was still green (which is generally just a combination of yellow and blue or brown and blue). They also suggest in their conclusions that this method may be useful for distinguishing natural and synthetic topaz which raises a few questions about how much background research they have done.
Regardless this one isn't that relevant to the question of detecting natural vs artificial irradiation, but I thought it was interesting.
More recently there is "Optical Properties of Irradiated Topaz Crystals," an open access paper by Skvortsova et al in 2015
This one is a bit cooler because they look at a Ukrainian colorless topaz before and after neutron irradiation as well as natural blue Ukrainian topaz and natural yellow topaz from India(!). The first downside is that I'm guaranteed to be unsatisfied by the phrase "yellow topaz" since that could mean light brown, golden yellow low-grade imperial or pegmatitic pure saffron yellow, but I'll try to let it go. The second is that I'm wondering where the heck they sourced actual topaz from India when all I've ever seen was mislabeled quartz, but they don't get into that either. The sample size is of course too small for any good conclusions--the spectra of the natural blue and irradiated blue are distinct but not so distinct that I'd bet money the distinction would hold across a larger sample size.
Next up "Photoinduced emission and thermoluminescence in topaz" by Dantas et al, 2006 in Nuclear Instruments and Methods in Physics Research B
This one is fairly technical, but while they do see a significant drop in the intensity of topaz's thermoluminescence after heat treatments (which is not restored by irradiation) they don't suggest any particular difference between natural and irradiated colored samples (the abstract says " Thermal treatments and irradiations do not produce significant changes in the TL emission"--not sure how this relates to what they said about heat treatment later in the paper though). I should also mention these samples were powdered for testing.
"THERMOLUMINESCENCE EMISSION SPECTRA OF GAMMA IRRADIATED TOPAZ" by Yukihara et al 1999
Looks intriguing but doesn't compare natural and irradiated, just presents various irradiated samples.
I got sick of reading these papers at this point. Sample prep generally seems to involve making small pieces of topaz from big ones and heating them to uncomfortable temperatures, and the results don't seem very promising. Maybe if you had a gem lab you could afford to run hundreds of samples to get something statistically meaningful.
Here's an intriguing title, however: "DIFFERENTIATION OF NATURAL AND IRRADIATED BLUE TOPAZ BY CATHODOLUMINESCENCE(CL) PROPERTIES", presented by Ying Song and Ying Qi at the CORALS II conference. All that's available is an abstract though, which suggests that natural blue topaz has much stronger cathodoluminescence than artificially irradiated blue topaz. I'm curious how big their sample size is. This one might be worth contacting them about.
Not relevant to the topaz at hand, but for a super blast from the past about early irradiation experiments you gotta read this paper, " EXPERIMENTS IN X-RAY IRRADIATION OF GEM STONES" from American Mineralogist in 1947 http://www.minsocam.org/msa/collectors_ ... emxray.htm It wasn't the first time stones had been irradiated by any means (they cite experiments from 1912) but it was probably the first time it was practical to do more casually because they finally had good doses of gamma rays to play with. Among other things they turned spodumene BROWN from long-lasting phosphorescence combining with the irradiated green (for a fun experiment, get a cheap violet laser pointer and an irradiated green spodumene. You can leave long-lasting glowing orange spots in the stone with the laser.)
Post subject: Re: GRS report for 'untreated blue topaz'
Posted: Thu Feb 22, 2018 8:44 am
Gold Member
Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
Didn't we have some day in the forum a discussion about the fluorescence of treated-untreated blue topaz ? If I recall correctly, we had noted that most irradiated topaz had fluorescence under UV light while natural ones had not. Or am I wrong ? Anyway this could hardly serve as a proof. Only an indication at most.
Users browsing this forum: No registered users and 21 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