This was done to submit to a customer who "designed it herself" ;) hence the weird look.
Anyway - the rings were modelled in Moi3d and rendered using one of Horo's HDRI's from the Gemstone Set. This render was experimental, really;I used a standard Diamond material but added Rainbow through the Specular Halo. For some reason I couldn't get any flecks of red into it, and the diamonds still look a bit lifeless.
I'd like to have them look bright but not like they're glowing (eg increasing ambience) - any clues, anyone?
Doesn't look too bad to me. Sometimes contras helps, if the ground is less bright, the rest appears to be brighter. Ambience is indeed not the best option. You could try to boost diffusion and specular with a hyper-texture. I have a PDF on my website that shows how such a texture can be made with a few clicks.