Hi Linda, I'm not much on fossils, but I have seen the type of iridescent ammonites you are referring to, and I do know a little about iridescence, so I'll offer an opinion. Sunlight causes colors to fade because the shortwave ultraviolet radiation in sunlight damages pigment molecules, altering their ability to differentially reflect light. In the case of iridescence however (the interiors of pearl oysters, abalones, some Turbo species, etc.), the multicolored display you see is not the result of pigments. It is the result of differential refraction (bending) of light rays, which in return is a function of the crystalline structure of the material itself. It's similar to the way a prism works, or the way a rainbow is formed. Therefore, I expect that sunlight should not cause any "fading" of the colors, since there is actually no colored matter (pigment) there to be damaged. I wouldn't expect light to alter the physical structure of the shell, and therefore I would not expect the iridescent effect to be affected. Regards, Paul M.