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 Post subject: refractive index and heat treatment
PostPosted: Thu Jan 31, 2008 7:50 pm 
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Does anyone know if it's possible for the RI of a gemstone to change if it has been heat treated? I think the answer is no but I wanted to ask anyway.


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PostPosted: Thu Jan 31, 2008 7:56 pm 
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you're right.... it won''t change


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PostPosted: Thu Jan 31, 2008 8:32 pm 
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Let's think this through... I bet we can come up with a few ways that it could change... if we're really creative...


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PostPosted: Thu Jan 31, 2008 8:34 pm 
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I've noticed a shadow effect on heat treated tourmaline, that one does not see on untreated stones.


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PostPosted: Thu Jan 31, 2008 9:14 pm 
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Let's also consider density when thinking of RI. Could heat treatment significantly alter the treated gems crystal lattice configuration to change the density and RI range for the gem?

Just shooting arrows at the moment. :smt017


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PostPosted: Fri Feb 01, 2008 1:41 am 
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I found a few abstracts of papers that seem promising for forming an opinion, but I don't have current research library access to read all. Most deal with radiation changes such as UV range, but a couple used heat to research RI and color change in halides. However the changes seem to be on the 10 to the -4 range such as .0003 or .0008 and from one longer blurb, seem temporary to the condition. Maybe the "cookers" of stones on the forum could measure RI before and after the heat for several stones and see if there
is any significant change. I really doubt any "measurable" difference can be validated.

Jim


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PostPosted: Fri Feb 01, 2008 10:07 am 
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I have always read that heat treatment can be difficult, if not impossible to detect. I now have a case of an aquamarine with a measured RI of 1.52 and the owner of the stone says that's because it was heat treated. So this would have meant about a .05 change in the RI. I don't believe this is possible which is why I originally asked the question.

Thanks for all the replies !


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PostPosted: Fri Feb 01, 2008 11:33 am 
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The owner is wrong.


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PostPosted: Fri Feb 01, 2008 11:34 am 
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The published refractive indexes for minerals represent a range from high to low and account for the varietal differences of the particular mineral species.
Common treatments such as heating and irradiation are accounted for within the published refractive indexes. If they weren't, we could no longer assume the listed indexes are accurate for any treated gemstone, including corundum, topaz, zoisite, quartz, etc. etc.

Aquamarine is commonly heat treated. The treatment does not exclude it from the known refractive index range of beryl.

I would ask the owner if he has a revised list of refractive indexes for commonly treated gemstones. :?


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PostPosted: Fri Feb 01, 2008 11:47 am 
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Hi,

The RI of zircon can change due to heat treatment and then there is of course the Kerez effect in some tourmalines that is associated with thermal shock.

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PostPosted: Fri Feb 01, 2008 11:53 am 
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Does it change or does it fluctuate within the published refractive index range of zircon.

(edited because I think Barbra thought I was asking about the aquamarine that I had already stated in my last post that it coudn't be aquamarine.)


Last edited by JB on Fri Feb 01, 2008 2:52 pm, edited 1 time in total.

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PostPosted: Fri Feb 01, 2008 12:03 pm 
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As I understand the latest question, can heat treated aquamarine have a refractive index of 1.52.

If that were possible, most aqua on the market today would have an RI of 1.52, as most of it as been heated.

Clearly, this is not the case.

When one heats an aquamarine, they are simply changing the oxidation state of a very small amount or iron within the stone.

Doos wrote:
The RI of zircon can change due to heat treatment

Please elaborate. What would be the change in RI? What causes that change? Since most of us can't measure the RI on zircon, we'd probably not have a means to check.


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PostPosted: Fri Feb 01, 2008 1:05 pm 
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Barbra Voltaire wrote:
What would be the change in RI? What causes that change? Since most of us can't measure the RI on zircon, we'd probably not have a means to check.

The range for high state zircons is 1.92-1.99, for low or metamict 1.78-1.90. The breakdown of ZrSiO4 to ZrO2 and SiO2 (excuse the number placement) from the thorium causes changes in the optical and other properties. I don't know about heating high zircons (to change the color) whether the RI changes, but medium ones can be heated and returned to the high state retoring the high state properties; the lowest ones are out of luck.

I was going to mention also high heat in corundum in conjuction with Cr raising the RI to OTL, but technically that is not simple heat treatment but rather diffusion.


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PostPosted: Fri Feb 01, 2008 1:07 pm 
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Hi,

Low and medium type zircon can be heated and become high type zircon again.
I have no clue about aquamarine, but I doubt that aswell.

We can check the RI by other means than the TIR refractometer of course.

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PostPosted: Fri Feb 01, 2008 1:19 pm 
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I'm assuming that there are some materials where heat could be used to alter the geometry of the atomic lattice making up the crystal form... presumably that would change the RI, no? Also, keep in mind that a stone could change it's RI (due to various causes) and still fall within the range for its appropriate species, right?


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