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 Post subject: OPTIC CHARACTER?
PostPosted: Sun May 20, 2007 2:50 pm 
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Hello it's me again! Can anyone tell me where to learn about optic character? I have read bits and pieces from my books, and looked around for a precise explanation.

I don't think I know exactly what I am looking for. It is probably right in from of me! :oops:

Thanks!


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PostPosted: Sun May 20, 2007 3:04 pm 
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TRY HERE, although it might be a bit advanced, there are pictures.


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PostPosted: Sun May 20, 2007 3:20 pm 
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Hi Breeze,

"Optic character" refers to the way light can behave inside a gemstone (or any other material).

The 3 optic characters are:
- Isotrope (light will travel inside the gem in every direction at the same velocity)
- Uniaxial (the velocity can vary in two different directions)
- Biaxial (there are 3 variable directions of light velocity inside the gem)

The 3 different characters all determine the reactions you will get from a refractometer, a polariscope and a dichroscope.

To make things more complicated, in anisotrope gems (uniaxial and biaxial gems) there is also an "optic sign".

Good pages to read are indeed the polariscope and the refractometer pages on the project, but can be very confusing if you're new to it.
Let us know if you still scratch your head after reading those pages and we'll explain better (or try).


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 Post subject: optic character
PostPosted: Sun May 20, 2007 5:45 pm 
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Thanks Doos..... I printed out those pages, and studied them, will study more. Haven't gotten chance to look at what africanuck posted, but will.

I have another question that may be part of the problem I am having with optic character, and birefringence.

I don't get a consistent reading on my refractometer. For example, I just looked at 5 different quartz, and I know they are quartz, and I get different readings on each... 1.54, 1.55 .... then on another 1.542, 1.599... 1.544, 1.552.... none of them match the quartz RI that I see in all the charts? I get a birefringence of 0.01, to 0.008 depending on which one I read??? What am I doing wrong?

I know for most of my gems, the readings are close enough to the charts to be quartz, tourmaline.. ect.... but they are hardly ever exact to the charts, so my birefringence comes up wrong. So wouldn't that make my optic character wrong also? :?


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PostPosted: Sun May 20, 2007 6:28 pm 
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Hi Breeze,

Getting a good reading much depends on proper use and lighting. Another very important factor in using the refractometer is pen and paper.
It is necessary that you take "full readings" on a stone. Which means that you need to record all the readings you get and then turn the stone 45 degrees, then record the readings again and so on (you've seen the example images).
So never base it on one reading.
You might also want to make sure the liquid droplet is not too large and that the stone is placed on the glass correct (on the dividing line).

For quartz the higher value will vary when you turn the stone (that is the extraordinary ray) and the lower value (1.544) will stay more or less the same (that is the ordinary ray).
If you have trouble estimating the 3rd decimal, you might try keeping your head steady and very, very slightly move your eye in a yes motion. You want to read the line where the seperation between light and dark is least vague (a sharp line).

On most refractometers you can slide the lens in and out for better focus, but dont push the lens down fast .. that may damage the tool.
A thing to avoid is to try and get readings when you are tired, that obscures your vision.

If you get weird readings, like 1.559 to 1.535 when you know it's quartz, then there is something going on that you need to eliminate.
When you get a 1.543 reading instead of a 1.544, then that is perfectly normal. The people who wrote the textbooks probably have not examined your stone and some slight deviations are more than possible. Or you are not reading correct .. if you keep measuring with the same mistakes then there is not much of a problem.


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 Post subject: optic character & refractometer
PostPosted: Sun May 20, 2007 6:45 pm 
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Thanks Doos. I go through paper like crazy...I ALWAYS take 4 readings, and usually more if I am uncertain. The "off" readings I am getting are consistent for each stone. I know that the estimated 3 # is hard for me...but I have tried to find a constant for myself.

I tested an Amethyst, Citrine, Smoky Quartz, Prasiolite, and then another Citrine to get an idea of what I was seeing. The highest -higher reading I got on any of those was 1.552... and of all my readings, I didn't come up with a birefringence of .009.... I guess that is why I am thrown off...

Is it important to get the birefringence of .009? Am I misunderstanding something? I am using the quartz as an example, but all my readings are alittle off like that, and so I quit calculating the birefringence because it never matched.

I just read something about the light used for the refractometer. I had to call for a replacement light for mine, and the one they sent was different. I really don't know how to tell what kind of light it is, maybe that is why my readings are off?


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PostPosted: Sun May 20, 2007 7:03 pm 
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Hi,

Quote:
Is it important to get the birefringence of .009?

No, if you get 0.008 then that is normal.

Quote:
I just read something about the light used for the refractometer. I had to call for a replacement light for mine, and the one they sent was different. I really don't know how to tell what kind of light it is, maybe that is why my readings are off?

You need a yellow lightsource/filter that emmits/lets through only the yellow portion around 589nm. You can easily check if your lightsource is good by looking at it with your spectroscope. If the spectroscope only shows a narrow band in the yellow, you have a winner. Else you can expect your readings to be off.
The refractive index that is mentioned in our textbooks is based on the yellow sodium light. When you use another lightsource the readings are different.


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PostPosted: Sun May 20, 2007 7:18 pm 
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my light is a yellowish greenish bluish color??? I don't see any bands thru the spectroscope at all.... just the rainbow...I went in a pitch dark room to make sure there was no other light, still no bands at all :x

Ok. I have a dark stone that is brown, with golden orange flashes of color in the light. RI readings of : 1.618 1.618 1.62 1.616
1.637 1.637 1.637 1.637
so RI of 1.616 -1.637 right?
With a birefringence of .021? is that correct? If so... it falls right in between Tourmaline, and topaz?

But the birefringence I get does not match either. I am so confused!! What am I doing wrong? :?


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PostPosted: Sun May 20, 2007 7:28 pm 
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Hi Breeze,

I have worded that all wrong. You should only see a yellow band (not a band in the yellow), sorry about that.
Your lightsource is obviously a wide band one (with some green and blue to the right .. or left) which is still workable.

Your readings indicate tourmaline, uniaxial negative with a birefringence of 0.021. Topaz will have two variable readings and the birfringence of topaz will hardly ever exceed 0.010.

I think you are using the refractometer correct, you are just a bit insecure. Stop being that ;)


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PostPosted: Sun May 20, 2007 7:41 pm 
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Okey Dokey Doos :P It helps that I had written tourmaline on the bottom of that gem!!! I was guessing for the most part, but at least I was right!!!! hehehe By the way, it is a beautiful gem! So much fire!!!

Now I know that you basically have to take the RI Charts with some leeway then, and rely on your other tests to confirm. I am still alittle confused on the birefringence... Am I correct that the birefringence also varies from the charts? And here again, I just take the readings, note them, and do my other tests to confirm my findings?

This is a negative because the constant readings are in the higher end?


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PostPosted: Sun May 20, 2007 7:55 pm 
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Hi,

The textbooks give "textbook readings". Lucky for us there are no textbook gems, otherwise we would all have to go to bed with nothing to pounder about.
The birefringence of tourmaline can go up to 0.039 orso.

Finding the optic sign for uniaxial stones from refractometer readings is the easiest thing in the world. You simply deduct the ordinary ray readings from the extraordinary ones.

The ordinary ray is always the constant reading.

In this case that is 1.616 -1.637 = - 0.021 (notice the minus sign).


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PostPosted: Sun May 20, 2007 8:08 pm 
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What about the stones that have variable readings in both the higher & lower readings? Are they the biaxial stones? And if so, which is the ordinary ray?

I have read & re-read the info on this, and just can't seem to get it :x

Do the different rays have anything to do with the pleochroism of the stone? Such as in Tanzanite? the 3 colors versus the 2? Or is that a completly different animal?


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PostPosted: Sun May 20, 2007 8:20 pm 
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Hi,

http://gemologyproject.com/wiki/index.p ... tical_sign

In biaxial stones, both readings are from extraordinary rays and your pen and paper become more important.

Quote:
Do the different rays have anything to do with the pleochroism of the stone?

It is all closely related.

Off to bed now.


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 Post subject: Optic char
PostPosted: Mon May 21, 2007 10:16 am 
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I wrote a small Windows program that will do this calculation for you. All you need to do is enter the values you read from your refractometer. If anyone is interested, I'll send you a link or if enough people want it, I'll post it on my website for download.

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Precison Gem
www.precisiongem.com


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PostPosted: Mon May 21, 2007 11:09 am 
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I am interested in any help I can get precision gem... I would be glad to visit your program!


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