I have a nice mid- blue sapphire which is a little on the shallow side: It doesn’t quite fit the cutting pattern I selected and I have to reduce the culet height; the TCP is set at 38.07; how far can I shorten that without interfering too much with the refractive index? The cut is ‘PC 02.191: Fancy Oval- Split Main Butterfly pavilion’ which I have already roughed in, but find that the cuts to the culet point aren’t leaving me much for the crown if I complete them.
You have several options here. I optimized this design in GemRay and the culet angles go up and not down for the optimization, altho’ the crown angles do go down about 5 degrees for the standard optimization. So you don’t gain anything there. However, this design is very bright and robust, so you could leave the crown as is and reduce the 30 crown angle to 25.95 without losing much in the way of brilliance. This would take your pavilion depth from about 0.555 to 0.530. You’d be reducing the crown height from 0.167 to about 0.141. If needed, you could go a little further than that. If you need less depth than that, you could reduce the star facets to something closer to 8 degrees and leave them as an apex crown. This would allow you to reduce your pavilion angles, even going below the critical angle (34.62) if needed. Some folks don’t like the non-traditional look of apex facets, so this could reduce the marketability of the stone, but maybe not.
I didn’t compute the design with star facets and lowered pavilion angles, but I could do that if you want to explore that option. It’s also possible that you could reduce the crown angle to 27 from 30 and the pavilion angle from 37.5 to 36. This takes you down to 0.516 depth. At this point, your design is still returning light all over, but is slightly less brilliant face on and in tilt. If the stone is not too dark already, this may darken it ever so slightly, which might be a good thing for the color, altho’ we’re talking very little here.
I think your course of action here has to be to measure how much existing height you have, allowing a smidgen for polish on the table, measure your width and estimate how much of that will be in the finished width, and then compute your total height. From there you can decide how much to reduce the pavilion and crown angles. My estimates show you can get away with 36 on the pavilion and 27 on the crown and you could go a little more on the crown. Beyond that, you could just shrink the width a bit of explore the apex crown options, which would probably give you an even flatter stone that still does not window.
I think this is a great example of how one has to proceed in these cases. Knowing how to tweak the cut will maximize your yield for these problem stones and you may even be able to bargain down prices for thin rough.
Awesome response and exactly what I needed- Thank you so much!