[Rocks][help] What is that mineral that is iridescent and slightly translucent and whose crystals are in brittle layers? There's probably more than one. But I need to know the name of at least one.

Opals?

opals are rainbow but its not naturally layered

mica?

talc also springs to mind!

Labradorite?

mica is probably what you're thinking of

Mica! That's it