There are already quite a few start ups out there offering customised 3D printed jewellery. Companies like Jweel, Nervous System, Blueberries or Polychemy give you the opportunity to design your very own and unique piece of jewellery.
Recently designer Miro Straka and Magpie Entertainment have released the free Android App called Ringz, that allows you to model and 3D print rings directly from your smartphone.
Ringz offers you a variety of models to choose from for further shaping and designing. The result can then be exported as OBJ file or uploaded to 3D printing service provider Shapeways.