Very quick one on getting sizes right for collets -

Click image for larger version.  Name: collet layout.jpg  Views: 55  Size: 32.2 KB  ID: 9100

The image is based on 17 degree collets, 0.8mm wall thickness and a 6.0mm stone. Left hand pair show a collet sized with the top at the same diameter as the stone, right hand the stone overlaps by about 50% of the wall thickness.

Getting the top of the stone level with the top of the collet prior to setting leaves about 0.5mm wall thickness on the left hand ones, whereas the overlap version leaves almost 0.2mm. This isn't thick enough to cope with much in silver... That, of course, is with a perfect bearing. If it's fractionally larger then you have even less metal to retain the top of the collet, let alone the stone.