The reason a diamond of that size poses issues is because it is LARGE. A 3ct round diamond is 9mm wide by 5.5mm tall. Normally a stone that size would be set in an intricate/classic/antique setting with lots of metal and supports. And to put that on a simple 2-3mm band is tough... it lacks stability. And it would need to be set very high for the bezel not to bulge out of the side of the band.
The first attempt was close, but not quite there...
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The client felt the stone was set too high (tall) for her taste and "v" for the half bezel was too small. But with some tweaking, it found the perfect compromise (the 2nd side view, on the right)...
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Now, I know you are thinking that this still seems like an easy project. But when dealing with such a large stone for a small size (size 6) it becomes difficult to keep the proportions in check. It's all about structure with a project like this.