Looking for information on cutting star corundum

[KbNeurobotX]

Hello everyone! Hope you are all enjoying this fine Saturday!

I have a quick question (probably very easy to answer for an experienced gem cutter/cabber who has (successfully) cut asteriated corundum.

I have a corundum that (based upon the very little information I found find on the Internet) I have cut into roughly a ball (ok oblate spheroid to be exact), in hopes of finding the right orientation to star, as the c-axis was not evident from the chunk of corundum. I found a couple places where someone recommended making a ball and then you’d be able to see where the star is. So I did.

What I have now (having ground off a significant portion of the stone to get it to a spherical shape) is a ball of dark purple corundum with what looks like a target or spiderweb of chatoyance on opposite sides (presumably that is the C axis?).

My question is: I have read that to properly orient the star in a star corundum, you center your cab on the C axis. I’ve also read other things that say to center it perpendicular to the C axis.

Before I grind away more of this stone, could someone please tell me if I center it on the “target” (assuming the center is the C axis)? Or if not how *should* I orient this stone on a dop stick for a proper star corundum?

Thanks so much for any advice you can give me!!

Best,
Kate

 

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[Clay Diggins]

Sapphire has a platy internal structure. That structure runs parallel to the C axis. It looks like in your picture maybe the plates are on "top"?

I always think of it this way. The plates will be parallel to the edge of the finished cab in a star corundum. The edges of the plates will be seen from the top of the finished cab.

More important is to figure out whether the stone will produce a proper star before you cut. Just rough polish a small window, clean and dry the stone and place a tiny drop of oil on the clean surface. With a single focused light over your shoulder in a darkened room look very carefully for a star to appear on the top of the oil drop. With careful observation you should be able to make out the shape and construction of the star. If it's a 6 ray, evenly spaced star you are right on your spot, if it looks more like a faint fuzzy line or flash you are 90 degrees off. If the star is complete but it won't center you are either off center or your stone has imperfections that won't allow for a good full star to be cut.

You've got some significant orange peel on your stone. That will remain a problem all the way to finish stage. Sapphires are like that. If you have properly orientated the star the top center of the stone will be less prone to orange peel than the sides. There are as many different methods of dealing with the orange peel problems with sapphires as there are lapidaries and that's probably a subject for a different thread.