Joined: Fri Jan 27, 2006 12:27 pm Posts: 764 Location: Western NY
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?
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
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.
Joined: Fri Jan 27, 2006 12:27 pm Posts: 764 Location: Western NY
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.
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.
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.
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.
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?
Users browsing this forum: No registered users and 11 guests
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot post attachments in this forum