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 Post subject: Re: Single Source Gems May No Longer Be Single Source
PostPosted: Sun May 13, 2018 12:39 pm 
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Marlow wrote:
Interesting....
Have your ever seen a very fine one from Madagascar, Namibia or Italy or Mexico in the color of the best russians stones.

Yes, i habe seen a few truly incredible examples from Namibia, leaning very blue on the green and exceptionally bright and vivid. Not typically though. As to the other question, it's a bit hard to say how it should be, but I think the cat is out of the bag on how it is used. Of course it helps that there is a tradeoff between saturation and dispersion, so a stone with less intense color could show more fire to compensate.


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 Post subject: Re: Single Source Gems May No Longer Be Single Source
PostPosted: Sun May 13, 2018 7:27 pm 
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I think the Russian demantoids are the gold standard to compare all other demantoids to.
Few, very, very, few measure up.


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 Post subject: Re: Single Source Gems May No Longer Be Single Source
PostPosted: Mon May 14, 2018 1:08 pm 
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What I want to say:

We have a wide color range in Demantoid now - not only the gold standard but many lighter colored yellowgreen, grayish green and and and....

From Namibia or Madagascar but Russia too.

Nobody in trade would call a similar colored grossular "tsavorite" or a Heliodor like the two crystals an emerald.

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 Post subject: Re: Single Source Gems May No Longer Be Single Source
PostPosted: Fri Jun 08, 2018 5:01 pm 
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Hello all,

My apologies for being so absent and not responding to e-mails. Life and work has been incredibly hectic these past several months. I have been working to finalize and produce a 'new' system for referencing color in gems. But more on that at some other time...

Regarding these 'single source' materials that are referenced on our website and that have been discussed here. I should have qualified the statement on our website and I will do so. This is a practical reference for commercially available, gem-quality stones of the type submitted to AGL. Meaning typically large and of very high quality. There are always potentials for specimen quality sources that are found elsewhere and we have many samples for the comparison of analytical data.

When one of these various gem species/varieties comes into the lab, we always compare the data to the subsets we have in our database. However because I didn't think it would be appropriate to charge for an origin determination, I offer it on these materials free of charge because of the lack of commercially significant material from other, more obscure sources. If we get data that is not clear, then we simply do not add an origin declaration. For samples like tsavorite which are more regionally associated, we actually utilize an origin of: East Africa, which includes Tanzania, Kenya, as well as Madagascar and elsewhere (not Canada of course...)

Speaking about the interesting topic of emeralds and its variety designation, here at AGL we use the dominant color-causing mechanisms (chromophore). The variety of emerald as defined here at AGL is a beryl where the dominant color-causing elements are Cr and/or V. Fe is virtually always present in trace amounts, as with Colombian, Afghani and other sources or as a minor element as may be encountered in sources such as Zambia, Brazil, Madagascar and others. A general description, and a bigger topic than can be appropriately addressed here in such an abbreviated fashion.

Some labs, gemologists, associations, etc. use the dominant observed color only as the determining factor for an emerald variety designation. Fe in beryl can also produce a green body colored stone. Think of much of the precursor rough to aquamarine which does not show a bluish color before heating... It is easy to utilize absorption/transmission spectra to determine if the body color of an emerald is due predominantly to Cr/V, Cr/V and Fe or only Fe. Here at AGL if the body color of a beryl is only due to Fe, we classify the stone as a 'green beryl'. In my simple opinion, this is an example of where the industry got it right to make a distinction between emerald and green beryl varieties however this should not be based on the observed color intensity but rather the dominant color-causing mechanism....

For beryl where the color is due essentially to only Cr/V, we do not consider the intensity of the color as important for the variety call however we do make an additional comment to explain why such lighter colored green beryl is still classified as emerald by AGL.

For beryl the contributing color-causing mechanisms are Cr/V and Fe, we do pay attention to the specific color of the stone submitted. If the dominant hue has transitioned to a blue hue, instead of a green one, we do not classify the variety as an emerald.

I always find the thoughtful and knowledgeable contributions made here to be very interesting and helpful to our gemological community at large and so I appreciate any comments made to or about what we do here at AGL.

Again, my apologies for not responding sooner and best regards to all.
Christopher P. Smith


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 Post subject: Re: Single Source Gems May No Longer Be Single Source
PostPosted: Fri Jun 08, 2018 5:08 pm 
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Hello again,

The same principle as applied to emerald/green beryl is applied to the demantoid/andradite classification here at AGL regardless of geographic origin. We have samples of what we classify as demantoid from a number of sources around the world.

Whenever possible, I have tried to develop an objective basis towards making variety calls and so on. This helps to greatly improve consistency and repeatability...

Again best regards to all,
Christopher P. Smith


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 Post subject: Re: Single Source Gems May No Longer Be Single Source
PostPosted: Fri Jun 08, 2018 6:39 pm 
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Excellent explanation.


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