Engagement rings, rings in general, and diamonds are all there own beast unique consideration. It is difficult to recommend a setup, or rather, a big responsibility to be in charge of saying "I think this is the best way to do it". There are a number of styles of photography in actuality. Engagement rings have been broken down into just a few styles for photography that you see in past "branded advertisement" images and every trend that has developed since.

The lighting setup you can have like the one described in this video and others, will work VERY well, despite the challenge.... It will not let you down. You will be able to take consistently GOOD "out of the camera" images, AND great retouched ones. 

 I think it is such a good setup, that I would recommend ANY piece be shot using it.