Saw a photo of optical glass sculpture with nice reflection/caustic within the glass body and I did some quick experiment to replicate the effect in 3D. I think the easiest method is to fake the total internal reflection with other smaller 3d models place inside the bigger model.
ArtworkThis is my test result, didnt really look like what I have seen. But still look pretty cool to be use as a diamond material and it render very fast as it is using Blender Internal.
Behind the ScenePut lots of small cubes inside the big cube. Then, enable Transparency and Mirror options in material settings. Putting a small point light inside the cube help a lot too.
You can also put a procedural cloud texture (colour) to point light to have this result.
Here is a suzzane render. This is why I said this could be a good method for rendering diamonds and gems with blender internal.
ReferenceDeviantArt Glass Cube
Blender3D Transmissivity Total Internal Reflection