My question is: what does 'upon' mean in the sentence:
He had come upon the diamonds one hot Sunday afternoon in January.
Thanks in advance
Not much, on its own - "come upon" is a phrasal verb. As curmudgeon said, it means he found them; but I think there's more to it than that - 'he found them by accident, while he was doing something else' Maybe he was going for a walk after lunch, or tidying the loft, or catching up on the filing ...