Couple of things are wrong within your file:
1. First off, you need to donload latest fixes for your Carrara: there is no
LightThroughTransparency option in your
GlobalIlumination panel.
2. Your
SpotLight is almost useless, since it's stuck on the floor, there is almost no point of using indirect lightning here.
3. Edges of the diamond need to be
ceased (go into modeling room, then select all edges and cease).
4. Follow the example of lightning in
Cornet HDRI scene supplied with your Carrara 5 (scene wizard), it is simple, yet, very good one, shows you using HDRI map as a source of global ilumination.
5. When you render glass like object, using caustics is highly recommended, to achieve more realism.
Good luck
