Thin transparent films produce rainbows - an effect due to the wave nature of light (the same effect that gives soap bubbles their rainbow colors). Here, the thin film might be photoresist or dried residue from some sort of solvent, like acetone. I’ll probably never know, since this wasn’t my sample - I found it abandoned in one of our research group’s storage boxes in the cleanroom. Judging by the giant broken edge at the left, and all the residue and particles on the rest of the sample, it was probably abandoned for good reason. Maybe they were using it to practice something on - I used it to kill time while waiting for the oxygen plasma machine to clean one of my own samples.