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?
Joined: Sun Jul 22, 2007 5:58 pm Posts: 294 Location: California desert
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.
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.
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.
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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Joined: Sun Oct 16, 2005 12:22 pm Posts: 21600 Location: San Francisco
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.
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.
Joined: Mon Oct 17, 2005 2:39 pm Posts: 3528 Location: San Francisco Bay Area, CA
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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