Blender is an excellent tool for modeling jewelry. It is great for intricate details and perfecting the shape you need. Blender can create high quality renders with lighting and also animate in 3D. Other CAD programs like Solidworks are great for engineers who need to know ultra precise details like material properties and dimensions and for automation.
As an engineer who has used both forms of computer aided 3D design I would recommend Blender for those specifically interested in designing and fabricating 3D printed jewelry.
The analogy goes something like: Blender -> CAD = Paint brush & canvas -> Ruler & pencil
And oh yea, Blender is open source and free!
Modeling Videos
A great & simple video on how to design a ring using Blender.
Creating effective shapes and textures:
3D Printing Preparation
After you have finished modeling, I highly recommend NetFabb for cleaning up your models, removing holes, measuring and prepping for 3D printing. It is free and easy to use!
Mocci 