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 Post subject: Polariscope and the C axis
PostPosted: Tue Aug 22, 2006 8:44 am 
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In faceting, it's sometimes essential to know where the c-axis of a crystal is. This is especialy key with topaz, which has perfect cleavage perpenticular to the c-axis. When you're dealing with terminated crystals, finding the cleavage plane is trivial.

With water-worn topaz crystals finding the c-axis is trickier since all of the crystal faces have been worn off. There are a couple of tricks to figuring this out, but one thing I keep being told is "just use your polariscope."

Now, I know how to use a polariscope to separating singly & doubly refractive stones, find an optic figure, and detect strain -- but I'm not at all certain how to tell where the c-axis is.

Anyone know how to use a polariscope for this? Am I just plain missing some basic theory here?

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PostPosted: Tue Aug 22, 2006 9:40 am 
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Peter,

In uniaxial stones the c-axis is parallel to the optic axis. Biaxial stones (like Topaz) have two optical axes that do not line up with the c-axis.
Topaz is orthorhombic with a positive optical sign, so you could get some general idea where the c-axis is by observing the interference figure with a conoscope and estimating where the Bxa is (acute bisectrix), as you will usually only see one of the isogyres.

I believe that the Bxa in Topaz aligns with the c-axis.

Alternativly you could drop the stone on a marble floor, the c-axis will be perpendicular to the cleavage plane :)


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PostPosted: Tue Aug 22, 2006 1:54 pm 
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pt,

A few additional tricks from another cutter. Concerning topaz.



http://www.faceters.com/askjeff/answer13.shtml


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PostPosted: Tue Aug 22, 2006 2:28 pm 
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One more link for those of you that may be new to gem cutting. This one has some nice tips on evaluating rough.

http://www.gemstoneartist.com/RoughFAQ.asp


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 Post subject: Topaz
PostPosted: Thu Aug 24, 2006 2:47 pm 
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Peter! What are you cutting topaz for????
It's not worth your time to be cutting topaz. Assuming you are working on blue topaz. Rough topaz seems to sell for about the same price as cut topaz.

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PostPosted: Thu Aug 24, 2006 3:44 pm 
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I hear ya PG -- and you're right. My question was mostly academic as I keep hearing other faceters say "just use your polariscope" and I couldn't figure out how it would help.

I do happen to have a bit of colorless topaz I thought about cutting up for practice as I haven't tried topaz in any form yet.

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PostPosted: Fri Aug 25, 2006 11:10 am 
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pt,

When they shout "use the polariscope", does anyone ever reply when you ask "how"?


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PostPosted: Fri Aug 25, 2006 11:45 am 
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The only salient answer I've ever gotten is "I think Vargas talks about it Faceting For Amatures". I haven't gotten my hands on that book yet.

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PostPosted: Fri Aug 25, 2006 12:14 pm 
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Would this be right?

If you are examining a piece of rough under the polariscope, the point of orientation where the interference colors appear should be within a few degrees of parallel to the optic axis.


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 Post subject: confessions
PostPosted: Fri Aug 25, 2006 12:27 pm 
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I must confess, when I first started cutting I cut some pretty cheap stuff. I bought some very cheap white topaz that was just as you described yours, water worn. I always just oriented the rough for the best yield, and never had any problems with a cleavage plane. Maybe I was lucky. Topaz can be a bit more problem than most stones to polish, and scratches more. I always tried to pick a design that didnt have a large table, or any really large facets for that matter.

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PostPosted: Fri Aug 25, 2006 12:30 pm 
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Quote:
the point of orientation where the interference colors appear should be within a few degrees of parallel to the optic axis

No idea what that means JB.
The center of the interference figure (the melatope) is the optic axis. In Biaxial minerals there are two melatopes.

pt,

I was asking because cutters don't necessarely need to be gemmologists and know what is happening in theory. But they do have a great deal of practical experience that alot of gemmologists lack in this field.
So maybe I missed something.

Yet in my simple logic, if you don't understand what is going on, you can not determine the c-axis in a biaxial gem by the use of a polariscope.

I could whip up some images if needed to visualize.


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PostPosted: Fri Aug 25, 2006 1:19 pm 
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Yes, whip up some images, please. :smt079


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PostPosted: Fri Aug 25, 2006 1:24 pm 
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Is the c axis an optic axis in topaz? If so when you find the interference figure, you are either looking perpendicular to a cleavage plane or parallel to a cleavage plane if it is the other(a or b) optic axis. Correct?


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PostPosted: Fri Aug 25, 2006 1:35 pm 
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Me and my big mouth, darned. I'll draw some crude images to illustrate.

In topaz the c-axis is not an optical axis.


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PostPosted: Fri Aug 25, 2006 2:41 pm 
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Quote:
I'll draw some crude images to illustrate.


Is it only me who gets nervous at the the idea of Doos drawing crude images? :lol:


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