Description: heat-treatment process - Nigerian Tourmaline

[Roger Dery]

Re: Description: heat-treatment process - Nigerian Tourmalin

This information is from a book by Richard T. Liddicoat, Jr. A well known gemologist. I think that what he is saying is that when most any stone is heated or treated a good appraiser or gem identifier should be able to confirm the process has been done. This is for beryl, tourmaline, or spinel, etc.

Unfortunately this book by Liddicoat aka The Father of Gemmology, was published in 1947. Liddicoat himself died in 2002. Gem treatments have moved on significantly in the last 9 years let alone in the last 63.

My understanding is that irradiation of tourmaline cannot be detected in a laboratory. As far as heating goes, I would be interested to know how high a temperature it can be heated to get a good result. I imagine that inclusions will not be altered at the lower temperatures and so the chances of detection are low.

Hi Pandora,
Re: heating Tourmaline
The highest I've heated Tourmaline, was to 750*C. It turned 'blackish' and looked toasted. It became less transparent, and was no longer green, but black-green. I did not plan to heat to 750*C, as this was in error. Costly error.

In my experience, there is little reason to go over 650*C. The vast majority of color alteration should take place between 400*C and 600*C. There is a large variation in the way they respond, and this is mostly due to content. Each mining area will have their own peculiarities, and then there is variation within goods from the same site. It is not like there is some perfect recipe - this technique takes time, risk, and a willingness to accept the failures.

Regarding inclusions, I have heated included Tourmalines. And, if, one takes the heating process very slowly, they may survive. But even then there is no guarantee.

Hope you find this helpful.

 

[Barrett]

Re: Description: heat-treatment process - Nigerian Tourmalin

Great thread, Roger. Well put together and useful info provided. I wish you and some of the other cutters would post more first-hand info like this.. It's wonderful to have direct evidence of techniques and treatments and the implications and results from them. The gem world is so full of "heard this or heard that" or "supposedly's". Great post!!!!!!!!!!

 

[cellentani]

Re: Description: heat-treatment process - Nigerian Tourmalin

Roger, interesting topic, and great photos! How do the two stones look in incandescent light? I've experienced a lot of lighter pinks like your second stone going a bit brown and muddy, and wonder if there's any difference between the two colors.

Cellentani,
So let's have a look under incandescent.....

These were all from the same batch, and generally their appearance before heating was somewhat interchangeable. On the left, heated to 525*C, center at 500*C, and on the right is unheated.

This photo was taken with 150 watt bulb at 18 inches. The Tourmalines are resting on a plate of clear glass, which is resting on a Photographer's 18% grey card. But as you can see, the incandescent lamp is 'washing out' all the depth of color in the stones as well as the surroundings. In the hand, there is a slight tan secondary to the stone on the left, almost giving it a 'peachy' hue. The center stone held its color fairly well, but the stone on the right has an obvious brownish modifier.

May be back later with further commentary.

Bummer that the brown didn't burn off! I know tourmaline has a complex, mile-long chemical formula, but what do you think causes the brown mask - is it trace elements such as iron? Funny how it's easier to burn off a brown secondary in other stones, like yellow zircon, for example. Also, Roger, have you ever experienced any drift in the tourmalines you've heated, where they've partially reverted back to their original color?

 

[Roger Dery]

Re: Description: heat-treatment process - Nigerian Tourmalin

Thank you Roger. I see a very distinct brown cast to the gems but I appreciate that may be more to do with the photograph, lighting etc. However, this is pretty typical of what I generally expect from pink tourmalines and is why I personally, am not enamoured with them. So I guess it's safe to say that heating doesn't drive off the brown or stop them shifting in incandescent lighting.

Hi LD,
The incandescent lighting certainly plays a part in the 'brownish' addition to the color. But even in my shop (fluorescent) lighting, there is still a faint tan appearance. The tan is most easily noticeable in the lightest of the pinks.

The control stone that I show in one of the photos, has a brownish-burgundy appearance that is more readily seen in the incandescent light - and then more of a purplish-red under the shop lights.

As you noted, "it's safe to say that heating doesn't drive off the brown or stop them shifting in incandescent lighting" -- however, I have had numerous pink Tourmalines over the years where there was no shift, or very little color shift going on. With the wide variety of color elements involved in the Tourmaline family, there is also a wide variety of results as well.

 

[Roger Dery]

Re: Description: heat-treatment process - Nigerian Tourmalin

Great thread, Roger. Well put together and useful info provided. I wish you and some of the other cutters would post more first-hand info like this.. It's wonderful to have direct evidence of techniques and treatments and the implications and results from them. The gem world is so full of "heard this or heard that" or "supposedly's". Great post!

Hey Jason,
Thanks... as you noted, the gem world is truly full of hearsay regarding treatments, or potential treatments.

There was a time when I too was of the understanding that heating would drive off the tan/brownish overtones to the red/pink Tourmalines. I am embarrassed to say that it wasn't until ten years ago that I learned differently.

I am hopeful that what I have shared will assist some consumers to make choices that are better suited to their needs, desires and pocketbook.

 

[LD]

Re: Description: heat-treatment process - Nigerian Tourmalin

Roger - thank you again for sharing your knowledge with us.

I understand that all pink tourmaline are not created equal My pet hate is when they shift to brown in incandescent lighting and so, for me, it would be a miracle cure if heating drove it off! Oh well!

 

[Roger Dery]

Re: Description: heat-treatment process - Nigerian Tourmalin

Roger, interesting topic, and great photos! How do the two stones look in incandescent light? I've experienced a lot of lighter pinks like your second stone going a bit brown and muddy, and wonder if there's any difference between the two colors.

Cellentani,
So let's have a look under incandescent.....

These were all from the same batch, and generally their appearance before heating was somewhat interchangeable. On the left, heated to 525*C, center at 500*C, and on the right is unheated.

This photo was taken with 150 watt bulb at 18 inches. The Tourmalines are resting on a plate of clear glass, which is resting on a Photographer's 18% grey card. But as you can see, the incandescent lamp is 'washing out' all the depth of color in the stones as well as the surroundings. In the hand, there is a slight tan secondary to the stone on the left, almost giving it a 'peachy' hue. The center stone held its color fairly well, but the stone on the right has an obvious brownish modifier.

May be back later with further commentary.

Bummer that the brown didn't burn off! I know tourmaline has a complex, mile-long chemical formula, but what do you think causes the brown mask - is it trace elements such as iron? Funny how it's easier to burn off a brown secondary in other stones, like yellow zircon, for example. Also, Roger, have you ever experienced any drift in the tourmalines you've heated, where they've partially reverted back to their original color?

cellentani,
I do not know for certain what element, or elements create/cause the brown mask in Tourmaline. That is probably a question for Chris Smith of Cap Beesley.

As for there being a 'drift' in color in Tourmaline, I do not believe I have witnessed this. To my knowledge, this is an occurrence known to be found in Zircons, but not all Zircons. If time allows this week, I may produce some Zircons and we can see what happens with them.

 

[lelser]

Re: Description: heat-treatment process - Nigerian Tourmalin

Here's the info for the May/June edition of Lapidary Journal/jewelry Artist. It has the article by me on Heat Treating Tourmaline and a faceting design by Tom.

I'm in the process of cooking a Brazilian indicolite now, so think good safe thoughts for it!

http://jewelryartistmagazine.com/archive/May-June-2011.cfm

Cheers,

Lisa
www.lisaelser.com
www.greengemfoundation.com

 

[lelser]

Re: Description: heat-treatment process - Nigerian Tourmalin

The Lapidary Journal edition with my heat treatment article is out of print. I've posted a copy on my website if anyone is interested.

http://www.lisaelser.com/f/HeatTreatment-JA_MayJune_2011.pdf

Cheers,

Lisa
www.lisaelser.com

 

[chrono]

Re: Description: heat-treatment process - Nigerian Tourmalin

Thank you!

 

[Alnitak]

Re: Description: heat-treatment process - Nigerian Tourmalin

I'm reviving an ancient thread here as I was reading through this today as I prepared to do some test heating for someone on some dark material they have. I wanted to add a few things to clarify matters on irradiation, heating and color as these threads live on and will be found in the future on Google searches.

Color in gemstones is complex; in tourmaline its particularly complex as tourmaline is sort of "garbage bin" mineral--its got all kinds of different things mixed up in it. This both gives it the wide spectrum of colors its known for and also complicates it in many ways as well, such as when irradiating and heating.

In general, however, certain things are known:

• The vast majority of the coloration in tourmaline is the result of Manganese (Mn) ions in different oxidation/reduction states.
  The bulk of other coloration is the result of Iron (Fe) and/or trace amounts of Titanium (Ti) or Copper (Cu).
  Differences in the crystal structure result in different color centers (places where these ions can be "trapped" in the crystal and therefore contribute to color); different color centers can product different colors from the same ions.
  All tourmaline is formed in a colorless state; the temperatures and pressures at which tourmalines form in the host pegmatites are so high that the color is not present, and the ions are typically not in the oxidation states necessary.
  Natural irradiation over the millions of years the crystals are underground results in the colors we see. The host granites these pegmatites form in are naturally radioactive at varying but low levels.
  The process of heating and irradiating tourmaline in labs roughly approximates the natural process, although on a compressed time scale and at lower pressures.

This is part of the reason we see certain colors from certain regions--the presence of different trace elements in the bedrock and pegmatites comes into play. In addition, pegmatites around the world vary greatly in age. Those in Brazil and Africa are some of the oldest known, the US is in the middle, and the Afghan/Pakistan pegmatites are some of the youngest. As one might deduce from that, we get deep red/pinks from Africa and Brazil, medium/hot pinks from the US and pale pinks and even achroites from Afghanistan and Pakistan. There are exceptions to this, of course, but as a generalization it works.

If you're interested in the science, there's plenty of articles out there, although its not easy to get them all. There's an excellent article from Kurt Nassau in American Mineralogist (Volume 60, pp. 71-713, 1975) and another great one by Ilene Reinitz and George Rossman in the same journal (Volume 73, pp. 822-825, 198.

In the interests of disclosure I'm a scientist by training and a gem geek, so I love reading journal articles more than the average nerd. I also know Dr. Rossman personally, and have chatted this topic with him at length. Were I younger, I would go back to grad school and study with him before he retires.

It is true that there is no really simple or cost-effective way of knowing if a tourmaline has been irradiated or heated or not. The process is so close to what naturally happens, its very hard to find any "proof" that its been done. I suspect for that reason one doesn't see a huge price differential in heated/unheated tourmaline.

Jeff

 

[Roger Dery]

Re: Description: heat-treatment process - Nigerian Tourmalin

Great post Jeff! And appreciate you bringing science into the conversation.

And so true that the price differential is low between treated - and - nontreated due to lack of proof and/or difficulty in finding proof. Am just now returning from Africa where there was very little Tourmaline available to buy from any of the various brokers we worked through. Very sad.... as it is one of my favorite gems.

On the bright side, there will be new material coming from the Democratic Republic of the Congo (DRC) in a few months. Will be mostly greens, green-blues and some pinks. But we must wait....