I just received another e-mail from someone asking about re-cutting a native-cut stone. Someone apparently offered to re-cut the stone, and:
"it would likely only need the pavilion facets straightened out, and the the bulges removed from underneath ... so the stone wouldn't even lose a carat".
Most of the windowing fat-bellied stones we see, the pavilion is roughly a hemisphere. That’s the bulges [sic] someone is referring to removing without losing a carat. Modern brilliant-style pavilions with “straightened out” facets are roughly cones.
So, to evaluate the “straighten out and remove bulge without losing a carat” statement, let’s look at some 7th grade math:
The formula for volume of a hemisphere is: 2/3 * pi * r^3
The formula for volume of a cone is: 1/3 * pi * r^2 * h
The volume of the cone will be ONE HALF the volume of the hemisphere, provided there’s sufficient depth to support proper angles without reducing the gem’s width – a luxury we usually don’t have. So, the yield is often less than 50% – a fact many customers find shocking.
Given that many native-cut stones are grossly lopsided, as well as too shallow to support their existing width with proper angles, a great deal more than 50% would be lost re-cutting many of them. Here’s a roughly profile-angle photo of the Sapphire re-cutting job you’ll find further down in the blog:
This stone was originally 3.5 mm deep, and we re-cut it to 3.2 mm deep, centering on the deepest portion, and losing only 0.3 mm in the process. The finished gem looks like this in profile:
To see the before and after of face-up appearance, have a look at this post.
Many faceters don’t keep a book on their actual yields, and wind up quoting insanely hopeful stories to potential customers. I recommend keeping a careful record of what you cut, and how you yield. Staying in touch with reality – and helping to improve performance are both very useful in my opinion.