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 Post subject: GIA Finds Synthetic Ruby Layer on Natural Gemstones
PostPosted: Mon Apr 02, 2018 11:51 am 
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GIA Finds Synthetic Ruby Layer on Natural Gemstones

Brecken Branstrator for National Jeweler wrote:
April 2, 2018

Carlsbad, Calif.--The Gemological Institute of America’s New York lab recently had a gemstone submitted for identification it hasn’t seen before.

According to an article by Tyler Smith and Hollie McBride in the winter 2017 issue of “Gems & Gemology,” the lab received two loose oval mixed-cut red gems for identification. Upon testing, the stones revealed refractive indices slightly higher than normal for corundum.

Higher refractive indices have been reported before for chromium-diffused corundum, in which extra chromium is added during heating to enhance the color. So, the article states, to the naked eye, the stones simply appeared to be chromium diffused and displayed red color concentrations at the intersections of the facets.

But further microscopic examination revealed a thin layer, about 0.3 mm thick, of “synthetic overgrowth”—a layer of lab-grown ruby that gave the stone its color—with a natural, almost-colorless sapphire at the core.

The authors said the two stones looked similar to Lechleitner synthetic overgrowth emeralds, named for Johann Lechleitner, who pioneered the process of giving a near-colorless beryl seed its green color with a synthetic emerald overgrowth in 1960. (The GIA’s lab in Carlsbad, coincidentally, received a Lechleitner emerald for identification around the same time the rubies were submitted.)

Lechleitner also experimented with ruby overgrowth on both synthetic and natural corundum seeds.

While his specimens contained a number of elements, the two rubies submitted to the GIA were missing one—molybdenum—indicating there were different growth conditions present, according to the G&G article.

It noted that while this isn’t the first report of synthetic ruby overgrowth on natural sapphire seeds, it is the first time material featuring such aspects has been submitted to its New York and Carlsbad laboratories for identification.

“The resurfacing of these vintage overgrowth synthetics shows that once a material is in the trade, it is here to stay,” Smith and McBride said.


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 Post subject: Re: GIA Finds Synthetic Ruby Layer on Natural Gemstones
PostPosted: Thu Nov 29, 2018 6:05 pm 
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Hi all,

I haven't been able to keep up with many of these threads over the past few months, as a result I just noticed this entry. For those interested, here is the original article on this material, which I published in 2002.

Best regards and Happy Holidays to all,
Christopher P. Smith


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 Post subject: Re: GIA Finds Synthetic Ruby Layer on Natural Gemstones
PostPosted: Thu Nov 29, 2018 6:28 pm 
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I wonder at what point this overgrowth should stop being called synthetic ruby or corundum--with chromium at structural concentrations and an RI way outside of corundum's range I would think it could be appropriate to consider it a different material (or a different mineral if it were naturally occurring.)


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 Post subject: Re: GIA Finds Synthetic Ruby Layer on Natural Gemstones
PostPosted: Fri Nov 30, 2018 10:25 am 
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Hi Stephen,

When I was first examining this material, I had a similar thought and so wanted to see if for example the Raman signature of the overgrowth had shifted from standard corundum. It hadn't. Also, we do sometimes record elevated levels of both chromium and iron in natural corundum, which can be individually above 1 wt%.

Solid solution series between Al2O3 and Cr, Fe, Mg and other elements have been proven and demonstrated experimentally. However, I am not certain at what stage/concentration/formulation it might no longer be appropriate to classify such a 'synthetic' as no longer corundum. However for the material we've analyzed thus far, it would still be appropriately classified as a synthetic corundum/ruby.

Best regards,
Christopher P. Smith


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