It is with great sadness I'd like to announce Dr. Hanneman passed away on December 12, 2020. His legacy will live on forever!
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 Post subject: ??Topaz refractive index??
PostPosted: Sat Sep 11, 2010 6:14 pm 
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I have this gem whats beleaved to be pink topaz i just took the refactive index and got 1.76 what could this mean ?? this is the gem in question
http://www.flickr.com/photos/zircons/4496106689
http://www.flickr.com/photos/zircons/4496744026
http://www.flickr.com/photos/zircons/4496743972

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 Post subject: Re: ??Topaz refractive index??
PostPosted: Sat Sep 11, 2010 6:46 pm 
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If doubly refractive I'd suggest sapphire.


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 Post subject: Re: ??Topaz refractive index??
PostPosted: Tue Sep 14, 2010 3:39 am 
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Dear! The RI's of Pink Topaz is 1.63 to 1.64.
I observed the doubling of pavilion back facets in the stone. So this is Sapphire not topaz.

Hamid Usman Khattak (HUK)

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 Post subject: Re: ??Topaz refractive index??
PostPosted: Tue Sep 14, 2010 11:32 am 
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Perhaps..perhaps not.
Sapphire has a low birefringence; I wouldn't expect to see doubling of the back facets.


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 Post subject: Re: ??Topaz refractive index??
PostPosted: Tue Sep 14, 2010 2:26 pm 
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My Dear teacher Barbra Voltaire!

You are right that the birefringence of corundum is low. I accepted that doubling of facet is not dominate in the case of corundum as comapare to tourmaline, Zircon etc but if you see the second picture of the post, you will observe that the backing facets junction looks double. There is a clear distance between the facets junction.
Teacher this is my personal opinion.

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HUK

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 Post subject: Re: ??Topaz refractive index??
PostPosted: Tue Sep 14, 2010 3:02 pm 
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I am more interested in your actual observations than what you observe in photos. :D


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 Post subject: Re: ??Topaz refractive index??
PostPosted: Tue Sep 14, 2010 11:29 pm 
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Barbra Voltaire wrote:
Perhaps..perhaps not.
Sapphire has a low birefringence; I wouldn't expect to see doubling of the back facets.

If I may state the obvious to those not trained to use only GIA approved techniques, looking for doubling of the back facets is a technique that is 100 or more years old. Times have changed. Do you hear me, GIA? :D

Today, one doesn't need a loupe, simply use the Hodgkinson Method, and you can numerically estimate RI, birefringence, and dispersion instantaneously. The very low Birefringence:Dispersion ratio is obvious in sapphire and diagnostic.


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 Post subject: Re: ??Topaz refractive index??
PostPosted: Tue Sep 14, 2010 11:58 pm 
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Clarification, please.
Are you saying, Dr. Hanneman, that one looks for doubling of back facets as a diagnostic characteristic of sapphire?
:?:

Really?

If we are attempting to separate topaz from sapphire, I doubt observation of doubling of the back facets will give us any clues at all.


But to pick up on your other topic, I think folks looking to identify unknowns with minimal investment in tools would be EXTREMELY wise to learn Visual Optics. It's not hard. I remember when I had my first VO party at my house (with some GIA Alumni). We had dinner with candles and everyone, including my son, who was 14 years old at the time was able to make some profound determinations, after about 5 minutes of practice. :D
PS Alan Hodgkinson was a guest at the party....that helped. :wink:


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 Post subject: Re: ??Topaz refractive index??
PostPosted: Wed Sep 15, 2010 2:53 pm 
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We both know one can't identify anything on the basis of one observation.
Doubling of the back facets is diagnostic for doubly refractive gemstones.

What I am saying is, Use of the Hodgkinson Method can be diagnostic. The doubling of the back facets, low dispersion, low birefringence, coupled with a Birefringence:Dispersion ratio of less than 0.5, and an RI between 1.7 - 1.8 . All of these properties can be instantly determined using the Hodgkinson Method, and combined they are diagnostic for sapphire—regardless of color.

Now, can you tell me even one advantage to be gained by using a loupe? :D

Of course, you can separate topaz from sapphire with your eyes closed. Simply squeeze it between your fingers. Topaz will slide out of your grip.

After rereading the above reply, I would like to add. The value of the Birefringence:Dispersion ratio should be taught to all students, even though I promote it. Do you hear me, GIA? :D


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 Post subject: Re: ??Topaz refractive index??
PostPosted: Thu Sep 16, 2010 7:33 pm 
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Wow iv been out of town for a few days i wish i had checked this post before now . I had no idea such great gemstone minds would be posting on my little topaz post but this is GO .A friend suggested that i use the LED computer screen and polarized sunglasses thing you know what im talking about and im allmost sure i found out its double refractive But becuase its so clean inside (no inclusions) it was suggested that it synthetic sapphire But i cant see striae So its very confusing to me im not a professional just a gem addict trying to learn All the other gems from this person have checked out no problem so... Can a lab gem have color zoning

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 Post subject: Re: ??Topaz refractive index??
PostPosted: Thu Sep 16, 2010 8:01 pm 
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I just did the Hodgkinson thing and there was NO green two rainbows and From top to bottom the rainbow was blue white a little yellow then orange :?:

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 Post subject: Re: ??Topaz refractive index??
PostPosted: Thu Sep 16, 2010 8:51 pm 
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Yes, color zoning can be in synthetic corundum if that is what it is. Topaz can be extremely clean though, typically, since so common, it is not typically cut unless really clean.

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 Post subject: Re: ??Topaz refractive index??
PostPosted: Thu Sep 16, 2010 9:15 pm 
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gemstoneJEFF wrote:
I just did the Hodgkinson thing and there was NO green two rainbows and From top to bottom the rainbow was blue white a little yellow then orange :?:

What you are seeing is the result of two overlapping "rainbows" and the result is white. This is charactistic for doubly refractive gems having low birefringence.

While this is more suggestive of corundum, try to estimate the birefringence:dispersion ratio or the RI by how large the circle of images appears, i.e. can you see more than two at a time.

Did you squeeze it?


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 Post subject: Re: ??Topaz refractive index??
PostPosted: Thu Sep 16, 2010 9:59 pm 
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swishman it does make sense for this to be synthetic sapphire but when i look at this stone in the darkfield then a synthetic sapphire they just dont look the same to me there is a very small chip on the faset junction near the botom of the pavillion

bill i just squeezed on it and also took out a irradiated blue topaz to compare the feel and the stone in question was not at all as slippery as the blue topaz i did it alot to and washed and dryed the stones and my hand to make sure and every time the topaz was always more slippery


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 Post subject: Re: ??Topaz refractive index??
PostPosted: Thu Sep 16, 2010 10:29 pm 
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Jeff,
There are different types of synthetic corundum, pulled/Czochralski process and Verneuil process/flame fusion for example. They both look different under magnification. ;).

You may not have syn. corundum at all, but the RI really points to it. And the fact that the stone is not slippery at all compared to known topaz is not a good sign either. P.S. I love that test, great for separating quartz and topaz in a hurry, lol.

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