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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PostPosted: Fri Sep 22, 2006 10:22 pm 
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Hi Annie and thanks for your feedback. I shall keep what you said in mind.

Thanks


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PostPosted: Tue Nov 28, 2006 11:11 am 
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Folks,

Hope that this is the right place to ask this question.

I have a parcel of mixed rough from Africa. It contains multiple types of stones, similar to what is sometimes advertised as Tunduru gravel -- alluvial stones of different colors.

One stone was a light pink, and was shaped just like a corundum crystal, with pryamidal ends. I cut the stone in a SRB with a high crown. The finished stone is light to medium pink with purple tones.

Ths stone is 6.3 mm wide and weighs 1.35 carats. The R.I. is 1.76+ and less than 1.77 The specific gravity is around 4. That is calculated backwards from the design of the cut, so it is an approximation but likely close. Other stones of known type calculate the S.G. right on with this method and the same cut design.

The problem is that both sapphire and almadine garnet have numbers close to these, with the garnet R.I. ranging from 1.76 to 1.83, while sapphire has a R.I of 1.76 to 1.77. Both have a S.G that could be 4.

The stone wanted to polish slowly with Alumina on the first couple pavilion facets, so I switched to diamond and polished the whole stone with that. Ordinarily this would be a good indicator of sapphire as usually garnet polishes well with alumina, but there have been times when a garnet facet or two did not want to polish with alumina and maybe I hit one of these first?

I assumed sapphire based on the crystal shape, but the fact that the facets would polish (though slowly) with alumina has me wondering a lot. The purple tones also have me uncertain, and the light pink is unusual for an almandine.

How can I make a determination of what this stone is? I have not been very successful determining S.G. via the weight in water and weight in air approach with stones this small.

Thanks,

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Bob K.


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PostPosted: Tue Nov 28, 2006 11:22 am 
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Garnet is singly refractive and sapphire is not.

Try using Visual Optics, as explained here:

http://64.233.161.99/search?q=cache:e8eQJqdS5gkJ:www.bovagems.com/eclectic/HTML/19980201_9802IDENT.html+peridot+refractive+double&hl=en

From that link:

Quote:
In doubly refractive stones you will be able to tell without a spectroscope the degree of birefringence. There is that terrible word which simply means the spectrum is partly or totally repeated. The clue here is to look at the red sector. In peridot you will see the red sectors are quite separate as they point towards the culet. With medium double refraction (birefringence) the spectra overlap and you see red reappearing part-way down the length of the spectra. In stones of low double refraction (birefringence) the second red can be very close to the first, or even be a lighter shade of red or pink. This is a very important part of identifying gemstones of the same colour; ie garnet, ruby and red CZ for example.


The technique is described in the article. As you can imply from the above, you will see a slight doubling of the spectra in a doubly refractive stone (sapphire), and with a singly refractive stone (garnet), you won't.


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PostPosted: Tue Nov 28, 2006 11:33 am 
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Also, based on the single vs. double refraction:
1. Polariscope reaction could set them apart. Beware of anomolous double refraction though! Test with coniscope for optic figure.
2. Dichroscope should be a useful tool. If the stone is dichroic it is not garnet.

Good luck!


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PostPosted: Tue Nov 28, 2006 5:15 pm 
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Folks,

Thanks for the ideas Africanuck. I have a version of the Hodgkinson method saved somewhere. I just have to find it. Hanneman also likes this approach. I was not aware of this specific approach which you were kind enough to point out, but it sounds like fun.

Barbra, I have a London Dichroscope which works well with larger stones, especially tourmaline, and have to better light this small stone. So far, it looks like the stone might have a lighter and darker tone through the dichroscope, but it's very small. I can only see it when i put the junction of the polarizers on a facet and then one side looks a bit darker than the other.

I was looking at Liddicoat's "Handbook of Gem Identification" and he says, on page 58 of the edition I have, "As a result (of cutting the stone so the table is at right angles to the optic axis) natural ruby and sapphire seldom show dichroism when viewed through the table..." As I cut the stone this way, maybe that is why the dichroism effect seems very small?

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Bob K.


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PostPosted: Tue Nov 28, 2006 6:01 pm 
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These stones are loose, right?
What is the approximate weight of the stones?

PS. If I had a nickle for every time someone misspelled my name, well, I'd have a really big jar of nickles. :wink: No worries!


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PostPosted: Tue Nov 28, 2006 6:52 pm 
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Barbra Voltaire wrote:
PS. If I had a nickle for every time someone misspelled my name, well, I'd have a really big jar of nickles. :wink: No worries!


How much would you have if you had a disme for every time someone misspelled nickel? ;)

Sorry Barbra, but I just couldn't resist!


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PostPosted: Tue Nov 28, 2006 9:11 pm 
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That's funny!
From now on, I'll make reference to dimes and quarters.


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