Step 1: Materials
Polymer clay: Super Sculpey is my favorite oven-cure polymer clay. It's smooth, soft, and easy to handle.
Liquid polymer clay: watered-down (so to speak) polymer clay. I use it here as an adhesive. It cures in the oven with the regular polymer clay.
Silicone Rubber: I used Smooth-Sil, available here: http://www.smooth-on.com/Smooth-Sil%3D-Plat/c1131/index.html. It comes in two parts, which need to be mixed in a predetermined ratio (by mass) and then left to cure overnight after pouring.
Modeling clay: I used an oil-based modeling clay like this one: http://www.dickblick.com/products/sculpture-house-prima-plastilina-clay/.
Mold release spray: If you forget this, you'll never get your mold halves apart. Available here: http://www.smooth-on.com/Release-Agents-and/c9_1123_1226/index.html
Step 2: The Plan
Step 3: Start Sculpting
Bake the form in an oven for 30 minutes at 275 degrees Fahrenheit. Check the clay packaging for brands other than Sculpey. I used a piece of rolled-up cardstock and some cardboard supports to help the ring stay round in the oven. I ended up stretching it a bit, however.
Step 4: Rough Sanding
If you want, you can continue sanding in stages through 220, 400, and 600 grit. I did, but I'm not sure if it really made a difference during the molding. I went all the way to 600 because I was afraid the silicone would adhere to the clay too much if I left the 150 grit scratches in the surface.
Once you've finished smoothing the surface, return to the 150 grit paper and sand off a flat to prepare for the diamond setting.
Step 5: Build the Stone Setting
Before you add any clay to the cured piece, brush on a bit of liquid clay to the building surface. Otherwise, the clay won't adhere to itself even after curing.
Build up the rest of the setting by adding more clay where it's needed. Then, bake the piece again and sand the new areas to refine the shape.
Step 6: Sculpt the Diamond
I actually had a hard time getting the diamond to stick to the setting - the joint kept breaking, and I never seemed to bake the liquid clay for enough time. If I did this again I would probably avoid trying to join two large flat surfaces with just liquid clay.
Step 7: Build the Mold Box
Form the clay in a boxy shape, with at least 1/4" of space around the edges of the part. The top surface of the diamond can be left exposed, because that's where you'll pour in the water later.
Once you're done with the clay, build the mold box around it. I used rectangular acrylic pieces hot glued together. The pieces should be glued to the base and at every edge. Run a bead of hot glue over the outside of every box joint, so that no silicone can leak through.
Use the back of some tool (or your finger) to poke holes in the surface of the clay. These will serve as registration keys to line up the mold pieces later.
Step 8: Pour the Silicone
Use the mixing ratio given by the packaging to determine how much of Part B (the silicone activator) you'll need. Re-zero your scale and carefully pour the correct amount of Part B. Mix them thoroughly with a stirring stick.
If have a vacuum degassing setup, you can remove most of the air bubbles from your mixture before you pour it. If not, you'll probably be okay anyway. Spray your clay, box, and model with the mold release spray, then pour the silicone slowly into the corner of your mold box. To minimize trapped air, allow the silicone to spread over the piece on its own.
Give your first mold half the proper cure time, then remove the box from the base and peel away the modeling clay. Clean it as best you can from the model, then give everything another good coating of mold release and pour the second half of the mold.
After both halves are cured, you can cast whatever you want. I secured the pieces together with acrylic pieces and rubber bands to provide even pressure and prevent leaks.