Very nice render !
I think that the geometry is very important to get nice dispersion/refraction.
As Octane is an unbiaised render, light follows the same path as in real world. Gems faces must be cut at the right angle, according to the IOR to allow full refraction of light. I mean that if a ray beam enter into the gem, if the angles of the faces don't create a perfect prism allowing the light to get out of the volume after N internal reflections, the gem will look dull, like a simple piece of broken glass...
So it is important to look at some references. There are many way of cutting a gem, and the angles and faces number can vary a lot, but there are precise rules, and they are different for each kind of gem (diamond, emerals, ruby...).
For example, a search with google with the keywords "diamond cut chart" gives many informations, and also an idea of the complexity of the task. :
http://www.google.fr/search?q=diamond+c ... d=0CCcQsAQ
French Blender user - CPU : intel Quad QX9650 at 3GHz - 8GB of RAM - Windows 7 Pro 64 bits. Display GPU : GeForce GTX 480 (2 Samsung 2443BW-1920x1600 monitors). External GPUs : two EVGA GTX 580 3GB in a Cubix GPU-Xpander Pro 2. NVidia Driver : 368.22.