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 Post subject: Re: garnet vs ruby
PostPosted: Mon Jan 27, 2014 8:30 pm 
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Thanks Barbra! Now I know that refractometer is off a bit. I'll be buying another one as soon as I can afford a good one!


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 Post subject: Re: garnet vs ruby
PostPosted: Mon Jan 27, 2014 9:23 pm 
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I think you were seeing a birefringence which didn't exist.
Remember to leave the polarized plate on when taking readings on faceted stones.
Rotate the plate and see if the reading changes.
Then rotate the stone.
Take readings again, rotating the polarized plate again.

Here's a little diagram I made several years ago:
http://www.gemologyproject.com/wiki/ind ... _gemstones


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 Post subject: Re: garnet vs ruby
PostPosted: Mon Jan 27, 2014 10:02 pm 
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I was just looking at another stone from the same group I had separated possibly ruby and I keep getting bad readings #-o Now I'm thinking this meter may not be any good for the higher r.i. type stones? I know it's accurate for lower types like topaz and amethyst and I've had good readings for garnets that were in the 1.780 area but not all of them I guess. The stones I sent you all read 1.770 as I saw it? So I don't know lol maybe the meter isn't always accurate or maybe I'm not doing something right. I use the polarized filter and turn the stones and take multiple readings but it seems like they only change depending on where I put the stone on the meter(closest to me, middle, farthest away I get no reading at all) and the glass the stone sits on is slanted down towards me and rises slightly further away. I think I'm done with this one, I'm just going to save a bit and buy a worthwhile refractometer.


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 Post subject: Re: garnet vs ruby
PostPosted: Mon Jan 27, 2014 10:27 pm 
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1.77 is not far away from 1.78.
If all your readings were singly refractive 1,77, I would have identified the unknown as Garnet, Almandine-Pyrope.

The spectrum was a dead ringer for rhodolite.


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 Post subject: Re: garnet vs ruby
PostPosted: Tue Jan 28, 2014 12:28 am 
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I don't know, it seems like it should be easy to distinguish but it isn't. I often get a secondary blurry area below the light/dark line and I don't know what that is? After the line it turns back to dark again then light which makes me think I'm reading lower values, but I can clearly see the light/dark line. I think a big problem is how the stones sit, they aren't always flush on the hemicylinder because it's slanted(they tend to sit on the edge of the epoxy/metal case). Would that cause problems like I described? Would a slanted hemicylinder also cause misreadings? Honestly I wouldn't mind spending a little money on something I know will work correctly, especially because I now have to go back and re-read everything I thought I had figured out. I probably spent a good 20 hrs. separating and making notes and double checking all those stones. Frustrating lol. Anyway I ordered a spectroscope so that should clear up the ruby/garnet issue at least.


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 Post subject: Re: garnet vs ruby
PostPosted: Tue Jan 28, 2014 12:55 pm 
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Perhaps you're over thinking this.
Bobbing your head around a bit more than necessary.

Get a faceted piece of quartz, like amethyst, a flame fusion synthetic ruby and a flame fusion synthetic spinel.

They will have RIs that are very consistent. The quartz and ruby will have a very consistent birefringence.

Knowing what the values ARE, bob your head until that is precisely what you are seeing with your refractometer. When you are able to get proper values for the 3 stones, you will know you are using proper form.

Make sense?


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 Post subject: Re: garnet vs ruby
PostPosted: Tue Jan 28, 2014 8:29 pm 
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Yeah absolutely, and I have done this with double refractive stones like amethyst,peridot and single refractive stones like the garnets. It just depends on the stone size/cut and how it sits on the meter. Sometimes I get perfect readings, sometimes no discernible reading at all, and like the case with some of the garnets mixed readings. Still I see I made a rookie mistake by not looking for birefringence in the garnets #-o stupid now that I think back on it... I will get some synthetics and practice on the ones I have already but I think this is just a "get what you pay for" type of thing. I will take a picture of the surface of the meter so you can see what I mean.


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 Post subject: Re: garnet vs ruby
PostPosted: Tue Jan 28, 2014 9:36 pm 
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:idea: Perhaps I'm misunderstanding, but garnets have no birefringence.


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 Post subject: Re: garnet vs ruby
PostPosted: Wed Jan 29, 2014 1:00 pm 
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Right and I should have noticed that right away, but it's blurry down at the bottom of my meter and looks like it could be birefringence sometimes. Does that make sense?


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 Post subject: Re: garnet vs ruby
PostPosted: Wed Feb 12, 2014 4:52 pm 
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faster way to separate garnet from corundum would be spectroscope, or dichroscope. Spectroscope is still best because it will also eliminate other possibilities.


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 Post subject: Re: garnet vs ruby
PostPosted: Thu Feb 13, 2014 12:32 am 
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cascaillou wrote:
faster way to separate garnet from corundum would be spectroscope, or dichroscope. Spectroscope is still best because it will also eliminate other possibilities.
Barbra Voltaire wrote:
I wouldn't reach for a filter either.
Refractive index (to identify the mineral) and microscopic observation (to separate natural from synthetic) is how you need to proceed. :D

Quote:
As I said, a month ago, "All the above is true. But you have a simple problem. Use a filter. Send a SASE to 264 Soledad Dr., Monterey CA. 93940 and I'll give you one.

Since this topic seems to be still a problem for many gemologists, let me extend my offer to everyone in this forum. :)


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