Usually, you start with a stone, and make a bezel to fit it. But in this case, I made a bezel to fit an odd-shaped cabochon, and the stone broke while I was setting it. So rather than throw the bezel in the scrap bin, I decided to practice making a cabochon.
I started by using the broken stone pieces to trace the shape onto a slab of agate, and used my Foredom with a diamond blade to cut the rough shape. You want to do this with a lot of water. I have a pet foot dish from the dollar store that I use to keep the stone wet while cutting it. It is also really important to wear safety glasses, because stone chips can still fly off, and you definitely do not want those in your eye!
After cutting the rough shape, I used the Foredom angle grinder handpiece in a mounting block set in my vice, to make a small flat-lap. The Foredom angle grinder comes with a screw-lok adapter, for which you can get a wide variety of abrasive disks. I have a set of diamond disks in different grits. These work well for rough shaping of the cab. Initially, I mounted the cab to a dowel with dopping wax and after it was closer in shape and I’d moved to the 400 grit disk, I just held it by hand. Again – you want to use water as a lubricant here, so keep the stone and the abrasive disks wet. This may throw a lot of “mud” all over, so probably good to cover tools and your workbench.
I frequently went back to my broken stone and checked the shape of my cab with both the stone and the bezel and marked my agate with pencil on the places that needed a little more work.
With the shaping completed, I moved to a combination of wet and dry sandpapers, and abrasive wheels on the regular handpiece, going up or down in grit as needed to get as many scratches out as I could and finishing with a 14000-grit diamond polish. the finished stone fits well in the bezel and is ready to set!