Post subject: Can a ruby have a AGG reading under the polariscope?
Posted: Mon May 19, 2014 2:06 pm
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I have recently purchased a loose untreated 1.05ct ruby. An identification report accompanied the ruby. Also I obtained a verbal second opinion from a local gemologist who also confirmed the identification. The ruby is orangey red with lots of inclusions which is an indication of it being untreated (no heat treatments or flux or lgf). I know that rubies should read DR under the polariscope blink 4 times on a 360 degree rotation. However under my polariscope the stone did not blink BUT remain lighted throughout the rotation which means its optic is AGG. I did a refraction reading and it read RI=1.76 and the spectrometer showed absorption pattern of a ruby. But because of the AGG reading I fearful that its not a ruby. Is it possible for a ruby to have a AGG reading.
I have tested lab created rubies (no inclusions) and LGF rubies with only a few inclusions but lots of bubbies due to the glass, in both cases the readings were DR. However the untreated ruby in question does have lots of natural inclusions. Would these inclusions effect the optic reading?
Last edited by JDBURKE on Thu May 22, 2014 10:32 am, edited 1 time in total.
Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Mon May 19, 2014 4:58 pm
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Joined: Mon Oct 17, 2005 10:56 am Posts: 6461 Location: The frozen north prairie :-/
When I took my GG courses, we were told not to test red or orange stones with the polariscope because they could give false AGG readings. The same was said for highly included stones whose inclusions could scatter the light.
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Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Tue May 20, 2014 6:06 am
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Joined: Tue Oct 20, 2009 3:34 pm Posts: 381 Location: Sweden
Barbara O. Ellis, GG wrote:
When I took my GG courses, we were told not to test red or orange stones with the polariscope because they could give false AGG readings. The same was said for highly included stones whose inclusions could scatter the light.
I was just going to write the same thing! That´s what I´ve been taught too...
Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Tue May 20, 2014 6:22 pm
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Joined: Mon May 14, 2007 11:00 am Posts: 1133 Location: Monterey, CA
JDBURKE wrote:
which means its optic is AGG. I did a refraction reading and it read RI=1.176 and the spectrometer showed absorption pattern of a ruby. But because of the AGG reading I fearful that its not a ruby. Is it possible for a ruby to have a AGG reading.
Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Tue May 20, 2014 10:09 pm
Gemology Online Royal Princess
Joined: Mon Oct 17, 2005 10:56 am Posts: 6461 Location: The frozen north prairie :-/
Barbra--In the distance ed course we were told not to bother putting any orange or red stones on the polariscope. I have looked in my Gem ID Lab Book under Polariscope, though, and I see where I wrote a note specifying "no red between 1.70-1.80 RI" and "no OTL stones", so the instructor must have told us that during the Gem ID lab class.
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Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Tue May 20, 2014 11:06 pm
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Joined: Mon Oct 17, 2005 10:56 am Posts: 6461 Location: The frozen north prairie :-/
Okay, I just put two rubies under the polariscope ... one natural and one synthetic. What I saw during rotation was the "brush stroke" reaction across the stones rather than a light-to-dark reaction (both of which are indicative of DR). So why we were told not to even bother testing the stones doesn't make sense to me .
(Lennie--Start testing your orange and red stones with the polariscope . We'll just have to be sure to use the confirmation test on ALL orange and red stones.)
That being said, though, JDBURKE's question was whether ruby could show AGG under the polariscope and the GIA manual says "yes". And from Liddicoat: "Sometimes corundum fails to extinguish light in the polariscope... Because of repeated twinning, some sets of plates are always in the dark (extinction) position, while the alternate parallel plates are not; hence the gem appears light in all positions." Also: "The polariscope reaction is not dependable for semi-translucent materials, in which the only light transmitted is a minimum amount through thin edges. Under these conditions, singly refractive materials often give results similar to those of doubly refractive aggregates, as do badly fractured stones and those full of inclusions." (Emphasis mine.)
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Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Wed May 21, 2014 10:49 am
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Joined: Fri Feb 07, 2014 4:50 pm Posts: 8 Location: New Orleans
Thanks to all for the responses. As a beginning student and novice, all of the information provided was valuable and helpful. The case of the ruby reading as AGG and the reasons how this can happen has definitely been a great learning experience. BTW, I requested my gemologist to take another look at the stone and after further test he again confirmed the stone as being a natural untreated ruby 1.08cts. and clarity VS2. It is unusual to find untreated rubies that are gem quality for $550.00 per carat. The ruby was purchased online from the National Sapphire Co. I checked into the company and it appeared to be legitimate and honest. The ruby was accompanied with a (in-house) gemological identification report confirming the identify and attributes of the ruby. On the report all the test performed was noted. The polariscope test WAS NOT performed. Since the report was prepared by the company and not from a independent lab, I was still skeptical and because I am a student I wanted to run my own test to confirm. After my polariscope gave the AGG reading, I immediately sent the stone to a local GIA gemologist who confirmed the identity and appraised the stone between $500.00 and $600.00. I then took the stone to a well respected jeweler to be set in a 14k rose gold diamond ring. Even though the ruby is included and has a orangey red color, the stone looks beautiful in the setting
Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Wed May 21, 2014 12:28 pm
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Joined: Tue Oct 20, 2009 3:34 pm Posts: 381 Location: Sweden
"If you are not very very skilled, you can misstake that phenomen to be a DR stone under a Polariscope so don´t test red stones under a polariscope" my teacher said. We tried that during some lessons and it was hard for us newbies...
Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Wed May 21, 2014 4:37 pm
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Joined: Mon May 14, 2007 11:00 am Posts: 1133 Location: Monterey, CA
Lennie wrote:
"If you are not very very skilled, you can misstake that phenomen to be a DR stone under a Polariscope so don´t test red stones under a polariscope" my teacher said....
That is ridiculous. There are too many red stones which can be easly identified on the basis of dichroism and optic sign to forgo testing with a polariscope. I would prefer to believe your instuctors meant to say, "Do not conclude a red stone (or any stone) is SR, DR, or AGG on the basis of only a polariscope observation.
Post subject: Re: Can a ruby have a AGG reading under the polariscope?
Posted: Thu May 22, 2014 5:14 am
Valued Contributor
Joined: Tue Oct 20, 2009 3:34 pm Posts: 381 Location: Sweden
Bill Hanneman wrote:
Lennie wrote:
"If you are not very very skilled, you can misstake that phenomen to be a DR stone under a Polariscope so don´t test red stones under a polariscope" my teacher said....
That is ridiculous. There are too many red stones which can be easly identified on the basis of dichroism and optic sign to forgo testing with a polariscope. I would prefer to believe your instuctors meant to say, "Do not conclude a red stone (or any stone) is SR, DR, or AGG on the basis of only a polariscope observation.
You are probably right on that! It´s a couple of years ago and my memory isn´t 100%.
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