I need to write you some tips, but I can't reveal all the secrets. I use latest photoshop CS6. That is it. No 3d programs, in fact 3D rendering programs usually look very fake.
YOu need to get good images. At least the stones need to be somewhat decent. You can send shank usually, but if the stones are messed up, then you have a problem.
YOu need to learn how to cutout the jewelry first. That means using the good old pen tool. Nothing else is as accurate as pen tool in Photoshop.