oops! my misteak! I used the term wrong. birefringence is two indices of refraction along certain axes, but (as ya noted) with diamonds the term I shoulda used is called dispersion - the ability to split up white light into its components. I see how your method should work, but i was hoping for a spectrum-type effect. dunno how to do it in carrara. although I also dunno how to apply anisotropic lighting, the term applies to a material that reflects light differently from sources at different angles. I reckon it also applies to materials that refract light in different directions, e.g. birefringent materials. I dunno if diamonds or other gems have that property.