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PostPosted: Tue Apr 10, 2007 1:23 pm 
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Hi VG,

I wouldn't go as far as to say this is a verdict or a myth buster. We all work with the best information we have available at the time. With the information provided in the GIA coursework, their Gem Reference Guide, from other sources like The Natural Sapphire Company and so on, we are led to believe that heating natural star corundum isn't very common.

Now, we are not talking about high temperature heat with the addition of chemical powders like titanium oxide to diffuse a star or synthetics or any of the newer treatments involving Be that Tom mentioned. Just plain old out of the ground corundum that contains enough silk, properly orientated to produce a star should the cutter find it more advantageous (profitable) than burning the silk out and producing a more transparent faceted gem.

Within that context, according to good sources, the lower temperature heating of this natural material to improve or create asterism doesn't seem to be common place. Being that is "undetectable" according to those same sources, it makes it difficult to form hard factual evidence other than the opinion offered by the experts.

I assume the experts that write the books, reference guides, and deal with these gems on a daily basis have done their due diligence when reporting treatment processes and when and how often they are used. Most of us are too far removed from the actual sites where the stones are mined and treated to make a judgement other than on "Good Authority" unless we have the ability to prove otherwise. In this case, "Undetectable" kind of limits our ability to dispute.

For the heck of it, I visited the AJSGems website. They are a large Internet colored stone dealer. They have many star sapphires and star rubies for sale. Everyone of them is classified unheated. Take a look.
http://www.ajsgems.com/


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PostPosted: Tue Apr 10, 2007 1:48 pm 
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The thread brought up a question about stars I was hoping to address one day. Rutile threads vanish with heat alright, but how about the hematite and ilemnite silk? Sorry for going back to basics, for some reason I don't remember to have read about this anywhere.


Last edited by valeria102 on Tue Apr 10, 2007 2:13 pm, edited 1 time in total.

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PostPosted: Tue Apr 10, 2007 1:50 pm 
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If this goes beyond the realm of "Beginners Only" we should certainly continue this elsewhere. I think it is a very valuable discussion that could potentially offer a substantial new point of information regarding treatment of star corundum (and other starred stones as well?).


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PostPosted: Tue Apr 10, 2007 2:43 pm 
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I wouldn't go as far as to say this is a verdict or a myth buster.

I agree. I should have said 'My" verdict, that is subjective and applicable to my needs. As a beginner I have a greater right for errors & misjudgments, jumping to conclusions :)
Well, I had this belief that a star corundum was always free from heating treatments, therefore at the purchasing stage I thought one less thing to worry as I wanted ABSOLUTELY a nontreated star corundum.(This was my subjective truth that needed to be verified, or called upon to pass a judgment)
Otherwise, NaturalSaphire Comp, or Lembeck, or AJS, are great companies and i still keep checking many others too, regulary....


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PostPosted: Tue Apr 10, 2007 4:58 pm 
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Hello,
As what I've seen up to this day star unheated star rubies are much more easy to find than unheated faceted rubies.
Now some start stones can be treated, the most common treatment I saw was glass filling. Of course there are also some diffused star sapphires but the good thing is that these treatment are possible to identify with a little bit of experience and this identification do not present any problem for any good gemological laboratory.

Regarding other types of treatment, it is difficult to say never but I've never seen any treatment like beryllium in this type opf stones. It is quite logic as these treatments are performed at tempearure much higher than the temperatures dissolving the rutile silk (responsible of the star) in the stones. Now regarding the possibility of a treatment to improve the quality of the star, some people have written about this possibility but I've never seen any evidence about it while looking at star rubies or sapphire using my gemological instruments or meeting people around in gem markets.

All the best,

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The views expressed here are V. Pardieu’s opinions and do not necessarily reflect those of GIA Laboratory Bangkok (http://www.giathai.net)where he is an employee since Dec 2008.


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PostPosted: Thu Apr 12, 2007 10:01 am 
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One never knows unless he is the miner so a simple disclaimer is probably wise. On the following quote

For the heck of it, I visited the AJSGems website. They are a large Internet colored stone dealer. They have many star sapphires and star rubies for sale. Everyone of them is classified unheated. Take a look.

I took a look and I think the line about being unheated for each stone was filled in for the first stone listed and all others got it as they used the first as a template. I say this because the pink sapphires are all detailed as being blue sapphires.

Best regards, Lee

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PostPosted: Thu Apr 12, 2007 10:31 am 
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Don't know what to tell you Lee. At least they put the pink ones in with the sapphires instead of the rubies.


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PostPosted: Sun Apr 15, 2007 1:45 am 
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Hi Frank. Thanks for contacting me.

This is what I've been told.

The star can be improved by heating to 1600 c and not above and then slow cooling. The star will be stable. No trace of treatment.

Diffusion treatment with titanium oxide heating to 1900c. Will enhance the color and the star.

How the stars are affected by newer treatments I don't know. I would listen to Tom and the other GG's on that stuff. Rom and Doos come to mind.

I'm digging through my old notes to see what more I can come up with. Also, have spoken to some dealers / friends about this and waiting for them to get back to me. Might be a long wait to get any good info and then confirm. But I'm on it. Busy


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PostPosted: Tue Apr 24, 2007 7:36 am 
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I've been informed that some heat treated rough star corundum can be detected, depending on the rough.

Also, as I thought would be the case. heating the star to improve the star can also improve color.



suggested reading.



Gemstone Enhancement History, Science and State of the Art by Kurt Nassau, The Heat Treatment of Ruby and Sapphire by Ted Themelis, and Ruby & Sapphire by Richard W. Hughes.


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PostPosted: Tue Apr 24, 2007 10:29 am 
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More recommended reading: 'Asteriated Corundums' pp. 49-54, first ed., "The Heat Treatment of Ruby and Sapphire" by Ted Themelis.

Heat can both eliminate silk and restore it, and can have major effects on color of star stones. Diffusion is another process for creating surface asterism and it uses heat.

I read somewhere (authoritative) there's a shortage of good star stones now because they have a limited appeal and it's more profitable to heat them clear blue for the mass market.

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PostPosted: Tue Apr 24, 2007 10:30 am 
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Hi Steve,

How do you handle heat treatment disclosure on star corundum then? From the vendor's standpoint.


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PostPosted: Tue May 01, 2007 10:16 pm 
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I just wanted to go on record that I always believed that whereas it's true that corundum can be heated to improve color the heating of a star corundum would disolve the rutile crystals and as a consequence of that eliminate the star.

That was my understanding. However, the GIA lab maintains, as I posted earlier, that star corundum can be heated to improve the star.

What can I say. (It wouldn't be the first time I got bum info from GIA BTW). That's what they tell me.

steve


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PostPosted: Wed May 02, 2007 12:18 pm 
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Beginner discussion or not, this discussion was a real eye opener for me as I believed that apart from diffused stones, Linde or doublets I would be pretty safe from treated stuff.

Ursula


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PostPosted: Wed May 02, 2007 1:44 pm 
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lembeckgems wrote:
The star can be improved by heating to 1600 c and not above and then slow cooling. The star will be stable. No trace of treatment.

Diffusion treatment with titanium oxide heating to 1900c. Will enhance the color and the star.


If you heat a star ruby or a star sapphire at 1600 degres with a slow cooling, I'm positive it will be possible in most cases to detect the treatment...

If the stone has some inclusions, they will be affected by heat and an experienced person will be able to detect it.

I've seen many rubies presenting some fine rutile needles which were also presenting some indications of heat treatment, most of the time the purpose was to make the stones a little bit less purplish (more red) and the increase a little bit its transparency. For star sapphire I guess it was also to increase a little bit the transparency.

I've some doubts about the techniques to improve the star. It's may be a possibility in theory, but these stones are not common. When they are found, the treatment usualy can be detected using standart gemological techniques by an experienced person.

Here is globably what is happening when a star ruby or a star sapphire is heated:

Titanium in corundum will have 2 different behavior depending the temperature: It can stay in solution or it can be expulsed from the host structure and form needles. Then there is the speed atoms are able to move in the structure: of course the higher the temperature, the easier the atoms will be able to move in order to form needles or not.

From room temperature to lets say 1000 degres, titanium solubility in corundum will be low and titanium tend to be ejected from the ruby structure to create rutile needles. Of course at high temperature these needles will form quite rapidly, at room temperature it might take much longer (centuries? more?)
Now over 1400 degres rutile needles will be dissolved as titanium solubility is high and the heat enable the titanium atoms to move "rapidly". At these temeprature if a ruby had some naturaly formed needles, these needles will rapidly diseapear.
Finaly from lets say 1000 to 1400 this is a domain were the 2 process are in competition.

So what's can we understand from that?

If you heat a star ruby at 1600 degres, all the natural looking rutile needles will be dissolved. To my knowledge they will never even if you cool slowly look like the fine rutile needles found in many unheated corundum: These rutile needles may present very fine arrow heads shapes and sometimes some small associated platelets (see last article by Richard Hughes ) These needles have formed sometimes over centuries... and their forms are very fine, detailed, precise...

Heating the stone at this temperatures for a long time enable the titanium to move in the stone and take a more uniformous distribution.
Then if you cool slowly the stone then as the titanium solubility will decrease, very fine rutile needles will form, the distribution will be probably better and the star might be better... as a result, ( may be...)
But these needles will be very small as the stone will cool during few hours or few days... I dont realy think that burners are colling them over few weeks as in this case the treatment will be to expensive to be economic. to do the thing good the cooling has to be slow to give to the titanium the time to regroup and form the needles. But I'm positive, these needles formed during cooling after heat treatment will not look the same as the fine natural rutile needles found in many unheated star stones. This is what I've seen so far and it seems also make some sense.

Possibly the star will be "better", shaper after treatment, but inlcusions will be different, silk will be different... In fact it will looks more like a diffused star or a star found in synthetic sapphires as the needles from these diffused star and synthetic sapphire are formed also by slow cooling.

So if you want to see the difference. Take an unheated star ruby and sapphire and then some synthetic stones, look at the silk carefully and compare: Then study the stone you want to buy, after a little bit of practice you should not make mistakes. Now if the needles are fine it does not mean that the stone was not heated at lets say 1000 degres, like in Blow pipe treatment to remove some purplish or increase a little bit its transparency of course... but this is another issue.

Hoping to have helped,

All the best,

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www.fieldgemology.org
www.conservationgemology.org

The views expressed here are V. Pardieu’s opinions and do not necessarily reflect those of GIA Laboratory Bangkok (http://www.giathai.net)where he is an employee since Dec 2008.


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PostPosted: Fri May 04, 2007 11:36 pm 
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great post. lot of good stuff here. I never sold a diffused star to my knowledge. with all the treatments going on I feel more comfortable saying possible heat. if i don't know. Some Ny dealers argue with me that the stars aren't heat treated. GIA says otherwise. And depending on the rough there might not be any "residue" after treatment.

If my source says definitely no heat then I can go by that. I have implicit trust with my sources as generally I know then 35 years.

Lee Little has a great post here that I can't find. He describes diffused stars. Geometrically perfect. Two dimensional floaters, appear on the surface of the stone only. Stars are "white" and appear well defined even in low light. Not familiar with diffused stars but I can say that - to further complicate things - there are natural stars that fit that description as well. I have some. So, what we all face is "know your supplier".

As for natural sapphire. I did business with his grandfather, Walter. Tried to do business with the kid but it never worked out.

BTW. Nice Star Ruby coming out of Vietnam. Very red.

Steve


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