You are correct about the refraction varying with wavelength; the formula above is a cheat to model that. I can't think off hand of a way to do real dispersion in Carrara other than to do multiple renders with slightly varying refraction indices and combine them in post. Might be tedious but for gem only pictures like you do, it might not be too bad. On the handle, it's amazing how seemingly always obscure names are already taken. Or my handle which is a combination of first and last name turns out to be a river in northern France. I've received mail several times from people wondering if I'm a relative (since the river name has become a family name). Marc