R.I. Angles

Hi everyone. I have a few questions about refractive index's and angles.

I have a large number designs that I have gleaned from the internet and I am trying to determine which one would best suit a large sky blue topaz I have, but some of the designs are more for heavily saturated stones like garnets and others are for very brilliant stones like zircon. Is it possible to pick a design I like then through some kind of "mathematical sorcery" find the best angles to suit the material I am trying to cut?

I have been mucking around with some smokey quartz lately. I have found that when I changed the angles to remove a flaw or keep as much material as possible the stones will often turn out either quite dull or very dark compared to one that is cut in accordance with the design.

I'm sure I could find the answer with Google but I learn better from others experiences then my own.

Total newbie to faceting here so take what I say accordingly

Every type of faceting material has a critical angle, below which the light striking the pavillion facets through the table will not be reflected back but will simply pass through and out - punch through the critical angle and the gem will have no sparkle. Referred to as a "fisheye".

Different materials have different critical angles. For quartz it's quite high, 40 degrees I think. Other materials have a lower CA which I guess implies that you can cut a lower, broader gem and still have it sparkle as a faceted gem should. The lower the CA the more leeway you have to play around with the shape of the stone heightwise I would think.

Our club has a list of critical angles on the back of the faceting cupboard, I think there is a list in an old faceting manual I have as well.

My auntie has some garnets that a mate of dad's faceted for her many years ago. He was self-taught and some of the stones show that he didn't know about the critical angle - the garnets are a nice, clear red (not over-dark) and look neatly cut but some are just fisheyes and have no sparkle whatsoever. When I examined them more closely, I saw that the pavillions of the dull ones were extremely flat and shallow. I'd say he tried to get the most out of the stones sizewize but went through the critical angle when he did.

Anyway, I hope that helps - someone with more experience will no doubt be along to offer advice, and correct me if I'm wrong

I cut topaz with a pavilion main angle of 41 degrees. The books will say 39 but this doesn't give you enough leaway for inaccurate machines and cutting mistakes you may need to correct with an angle drop.

listen to what Anthony said

The two last set of numbers on the far right have a heading ,"pavilion ... Main and girdle/break" ...just put this in because its too dark too see .