Just some food for thought, off the top of the Make home page: http://makezine.com/2012/04/05/how-t...-refractories/

I've heard Rhino (???) mentioned before for jewelry making, but I am unfamiliar with it, or other options. I'm a one trick pony when it comes to design work, and Sketch up could work for you, but I doubt it.

There's a 3d design sub forum here somewhere, that'd be the best place for this. IM we have way too many sub forums, and cause of that, stuff gets tossed every which way.

One thing about shapeways. If you're going to be printing the molds them selves, then using a 3rd party printing service may be your best bet. But if you're going to be printing a prototype, and then casting a mold around that, you might be ahead financially to buy a small printer ( I can't see you needing 8" bed sizes) yourself. The cost of a half dozen Shapeways prints might add up to the cost of a small printer.