Iridescent shader was one of the first guides I ever created for Corona. Back then it was possible thanks to a Corona Color Mapper node, which was later removed. Here is an update to that, which can produce exactly the same results.
The main idea is to use Gradient Ramp node with type set to "mapped" and falloff used as "source map". You can then create a rainbowy gradient, and plug this into reflection, diffuse, or any other material component you wish.
It can be also used to produce a kind of fake dispersion, though it's _a bit_ slow.
A sample max scene can be found below.
(https://forum.corona-renderer.com/index.php?action=dlattach;topic=14552.0;attach=57395;image)
(https://forum.corona-renderer.com/index.php?action=dlattach;topic=14552.0;attach=57397;image)
(https://forum.corona-renderer.com/index.php?action=dlattach;topic=14552.0;attach=57399;image)