Soap bubble-like rainbow colors means that this sample has a thin film of something on it, just like a soap bubble or oily puddle.  The colors change quickly, so there’s probably a ton of variation in the thickness of the coating - in fact, the rainbow stripes are packed so closely to one another that you can only see the colors under a microscope - to eye this sample just sort of looks a rough grey.

I have no idea what this sample is, or whether it was ever supposed to look like - someone left it behind, unclaimed, when they graduated.  But probably it was never supposed to look like this… natural-looking features usually means that some chaotic effect has taken over.

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