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.