I would go so far as to suggest that it is not merely behavior confined to scientists. Any potentially valuable item sent unsolicited to a stranger is unlikely to be returned.
People expect better of scientists, but shouldn't.
If you want to keep your rock, pay for an appraisal.
Yup. Even if the scientist wants to give it back their employer has a high likelihood of getting involved and behaving how faceless bureaucratic entities behave.
I don't have any skin in this game, but how's the saying go? Possession is 9/10s of the law?