Our friends at i.materialise have put together their own list of cheap or free 3D CAD programs to help people get started with product design CAD for 3D printing.
i.materialise Article – 9 Free CAD Programs To Get You Started with 3D Printing
While not all of these choices are the easiest to learn (Blender has a reputation for a steep learning curve), and none of them are particularly designed for making jewellery specifically, it is a good list. But if we’re discussing inexpensive CAD programs for 3D printing, I would also add 3DCoat as well.