I make renders of the gems in Carrara 2.1, and use them as texture maps. :) my gallery has quite a few examples of my gemstone Carrara renders... hence why I posted the link to one of the images earlier up in the thread. Carrara is perfect for this - it has an addon fresnel plugin and utilises caustics, making the renders very realistic. The upside is that the render matches the gem model geometry, too.