Don't know about a gravestone,however, we live in Az. and travelled across country this past summer/fall to visit my brother in Ohio. Out west, there were a few caches in cemetaries, but in Ohio where he lives there are Spirit Runs and EVERY cemetary around there has at least one cache. It was GREAT! Check out a cache called 'City of Angels' near Little Italy in Cleveland. It is a HUGH cemetary where Rockefeller and President McKinley are buried. Now THAT was a hunt! We sure liked the cemetary caches since it was like a 'walk in the park' compared to the cactus around here
We found one cache (not in a cemetary, but would probably work for you) where there was a hole drilled into a post and then there was a reflector attached to the lid~so you had to unscrew the reflector to get to the log in the hole. Might work in a gravestone!