Here's something like mother of pearl.

I'm using the specular node with very high roughness to pick up the main light direction. The iridescence is then driven strongly by your lights.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)