You probably have a diamond on your hard disk already! If not there are lots of free diamonds on the net! For instance here,
http://www-c.inria.fr/gamma/download/CRYSTAL/index0.php

or here,
http://sketchup.google.com/3dwarehouse/search?q=diamond&btnG=S%C3%B6k&styp=m

The models at 3d warehouse are free to use, even commercial!

Or, to be frank, to model a diamond must be the easiest thing in the world.

Once you have the diamond, just open it from inside Photoshop CS4 Extended and pick up the diamond, turn and scale it to your satisfaction.

In this way you will get the best result, no talk about it.