I'm still happier with the refraction argument I mentioned, instead of this diffraction one. Diffraction depends solely on the periodic physical layout, not on differences in refractive index. An optical diffraction grating involves only rulings, not a medium. Diffraction is primarily a phenomenon of transmission, not reflection, although I'll admit reflection effects are also possible. Oil slick on water is the most common experience of iridescence.
Iridescence is a common phenomenon on cooked, sliced ham.
Blotting the surface removes the effect. The muscle fiber alignment remains the same. Robert A. LaBudde, PhD, PAS, Dpl.