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 Post subject: Bi-color tanzanite?
PostPosted: Sat Oct 06, 2012 11:40 am 
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Hi,

Image

Does it look like decent and perhaps collectible bi-color tanzanite? ( I have no clue how it should look like cos I never saw one) Details @ multicolour.com

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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sat Oct 06, 2012 1:27 pm 
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It would have appeal to us collectors, & David is always a great person to buy from. I'd say yes, if your not looking at the mainstream, but the small niche of collectors who can apreciate it!

BTW, nice stone! :D

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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sat Oct 06, 2012 2:37 pm 
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I'm with Elvis.
Multicolour is an excellent source for gems.


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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sat Oct 06, 2012 11:00 pm 
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It's like with all gems, personal taste.
Personally I think it's gorgeous.
There's plenty love bi coloured gems like tourm, ametrine I've even seen some gorgeous bi zircons.

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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sun Oct 07, 2012 7:42 am 
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Thank you for all the feedback on this matter. I guess I will try it and see if my client likes it in the ring. Apparently, she was very disappointed with the bi-colour tanzanite from Gems TV. Claimed that it was nothing special. I also read somewhere that most tanzanite's are heated to enhance their colour and it can produce bi-colour tanzanite, but Multicolour.com states that this is an untreated stone.

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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sun Oct 07, 2012 12:03 pm 
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I also read somewhere that most tanzanite's are heated to enhance their colour and it can produce bi-colour tanzanite

Where did you read that heating produces bi-color?


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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sun Oct 07, 2012 9:15 pm 
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fameojewellery wrote:
Thank you for all the feedback on this matter. I guess I will try it and see if my client likes it in the ring. Apparently, she was very disappointed with the bi-colour tanzanite from Gems TV. Claimed that it was nothing special. I also read somewhere that most tanzanite's are heated to enhance their colour and it can produce bi-colour tanzanite, but Multicolour.com states that this is an untreated stone.


Maybe I'm very partial to David Weinberg & Multicolour, but if he says a stone is untreated, I'd say 99.99% of the time, it is.

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 Post subject: Re: Bi-color tanzanite?
PostPosted: Wed Oct 10, 2012 1:40 pm 
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Barbra Voltaire wrote:
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I also read somewhere that most tanzanite's are heated to enhance their colour and it can produce bi-colour tanzanite

Where did you read that heating produces bi-color?


I think it was either GemTV or Etanzanite.com website, but can't find the article. Maybe it was part of stone description.

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 Post subject: Re: Bi-color tanzanite?
PostPosted: Thu Nov 01, 2012 4:11 pm 
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I have yet to burn Zoisite so only speak from studies:

The rough brown Zoisite is most commonly heat treated to produce blues...Than secondary heating is sometimes done to further reduce the green/purple/red elements...Most rough is prolley treated well before it's ever marketed...

The few blue specimens that have been unearthed prolley underwent a secondary heating in nature after crystal growth...

What we do as burners for the most part is just copy what nature has already done...We can tweak it here and there to achieve different results which are usually detectable...Not always...


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 Post subject: Re: Bi-color tanzanite?
PostPosted: Thu Nov 01, 2012 5:15 pm 
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I've heard that the out-of-the-ground blue is probably just a result of natural heating. It makes sense, but why does the artificial heating produce a less pleochroic gem? Could it be something about the temperature or duration?


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 Post subject: Re: Bi-color tanzanite?
PostPosted: Thu Nov 01, 2012 6:21 pm 
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I had not heard that prior to reading this thread...

There are many components of Nature that are not always duplicated by burners...For instance pressure...Unless a burner has a very expensive heat treating furnace he can't duplicate that environment...So how many tons of rock were at one time on top of the crystal while growing or during secondary heat??...This could cause a different effect??

Soak time and cooling time...We are somewhat hindered by time due to the expense of operations while nature is not...What was done in the ground over very extended periods of time...Perhaps years...We try to accomplish in a few hrs or days...This too can have an effect...

Gas environment....We don't know what that was in nature some thousands of years ago when a crystal grew or had secondary heat...We can only speculate and experiment with gas atmospheres but most burners are very primitive and don't even attempt anything more than heat by fire in air...This too can have a huge effect upon the final color...

What type of secondary heat treated those crystals in nature??...Geothermal...Hydrothermal...All of which could also have an effect upon the gas environments...

Usually in heating what we do is affect the chemical composition is some way...Take elements out of solution or put elements into solution...Sometimes affect changes in the latices structure...All or which can have an effect upon the color that our eyes see...The chemicals/gasses/fuels used in burning can all affect the final composition and thus color/clarity...

As to being less "pleochroic" that may be very debatable??...The colors can become less noticeable...Harder to distinguish perhaps...Present less influence on the color our eyes see...But in nature no two crystals are exactly alike and no two deposits grow the same either...So what one sees in one crystal affected by heat may not correlate to 10 others waiting to be heated...


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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sun Nov 04, 2012 11:00 am 
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Cut ThroatTraders wrote:
I have yet to burn Zoisite so only speak from studies:

The rough brown Zoisite is most commonly heat treated to produce blues...Than secondary heating is sometimes done to further reduce the green/purple/red elements...Most rough is prolley treated well before it's ever marketed...

The few blue specimens that have been unearthed prolley underwent a secondary heating in nature after crystal growth...

What we do as burners for the most part is just copy what nature has already done...We can tweak it here and there to achieve different results which are usually detectable...Not always...


I don't cut a lot of tanzanite, but do buy the rough. I have purchased rough tanzanite both at Tucson, over the internet, and in Arusha Tanzania. I have never seen heated stones for sale. All the rough I have seen has been unheated.

My findings are that green doesn't burn away. What I look for is that root beer brown with gray. These seem to heat to the best colors. A strong green on one axis don't heat well.

I do often buy natural blue stones when available. These to me, show the 3 colors stronger than the stones I heat. Again, my experience with tanzanite is not thousands of stones, just a few hundred. Rough dealers will ask a premium for any fancy colors however, and the bi-colored stone above I think would draw a premium.

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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sun Nov 04, 2012 11:29 am 
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Thanks for that Gene....I think you are correct.
I found an old G&G article which pointed out that there is a compositional difference between the blue/purple areas and the green areas.
Full story available here
Gems & Gemology, Spring 92 wrote:
EFFECTS OF HEAT TREATMENT
The violetish blue color commonly associated with tanzanite is usually the result of heat treatment, at approximately 500°C, of zoisites with strong brown to yellow modifiers (that is, the conversion of V3+to V4+;(Schrnetzer and Bank, 1978-79).The authors subjected the five groups of rough gem-quality green tanzanites to temperatures up to 650°C. The seven stones that were dark green to medium slightly bluish green down the c-axis showed no change in color. This lack of response is most likely because the color in these stones is due to appreciable amounts of Cr3+ ,which does not respond to heat the way V3+ does. However, the remaining 33 stones did change on heating.
Specifically, tanzanites that showed bluish green to brownish green responded to heat treatment by turning greenish blue. This corresponds to the response reported by Schmetzer and Bank (1978-79) for bluish green zoisites with nearly equal amounts of chromium and vanadium. Even though our research showed that dark bluish green to medium brownish green tanzanites will respond to heat treatment, our experience is that such enhancement has not been routinely practiced on such stones thus far because of the rarity of the green color and the risk of damaging stones that have inclusions. From this research, we also know that not all green tanzanites will respond to heat treatment, and it appears that a "pure" green color can only occur in nature.


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 Post subject: Re: Bi-color tanzanite?
PostPosted: Sun Nov 04, 2012 1:26 pm 
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@Cut ThroatTraders: thanks for the quick rundown on heating. The pleochroism thing is more something I've heard (mostly around here, I think) than observed over a large body of stones. I've owned a number of crystals, but certainly not enough to get a statistically significant sample.


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 Post subject: Re: Bi-color tanzanite?
PostPosted: Tue Nov 06, 2012 1:34 pm 
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Precision Gem....

Interesting...Not what has been reported here in Thai but not everything I hear is true...

Much of the rough that comes here has already been treated in some way while sold as untreated so a buyer must stay on his toes...They even oil/epoxy penetrate some rough other than Emeralds to hide cracks...Red oil for Ruby...Opal is soaked in water and brought to the market looking fresh and not left out for long so it doesn't start to dry out among many other tricks...There have even been reported cases of rough dealers carving synthetics to look like rough or spiking an old claim with them not to mention all the cosmetic changes Jadeite may go threw to make it more appealing...Then there's the quenching of synthetics followed by heat to heal feathers and even Be to make your eyes think they know what your looking at which is NOT restricted to only faceted stones...It's all about the $$$$'s and anything is fair game to get a buyer to give them up...A view of the net and the general market place is enough to make anyone hesitate...I had never been around such a fraudulent industry before entering the gem world...I have to admit to being very skeptical almost always about almost everything...Not much that I trust in this venue...

There are also very reputable rough dealers and the majority of the sapphire/ruby comes in untreated as does many other types of rough...But the moment a dealer (Rough or Cut) advertises "Untreated" I get very suspicious and take a longer look...Problem is NOT all heat is detectable and they know it...Unless an included crystal exploded or fractured how would you tell if or when a Zoisite was heated???..Now add in the fact that a blue one will fetch greater $$$$'s than that brown and _________!!!...Sometimes my attitude is to just assume heat but not all assumptions are correct either...

A study of treatments to a very large degree is a study of how fraud tricks you out of your $$$$'s...Unfortunately we are all very susceptible to being fooled by our eyes/hearts/minds...


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