When I was much younger we went on a school visit to a nuclear power station. They had a Geiger counter in the operations room they they used to demonstrate how non-radiationy the environment was.
They did all the usual bits, plus pointing it at various kids watches and things, but when the chap pointed it at a piece of Cornish granite the thing went absolutely wild.
We used to go to Cornwall every year on holiday and go up at least one granite tor while we were there. Might explain a lot...