That kind of makes me think that you have a smart puffer...you see puffers have unique teeth that are constantly growing and if you do not feed them hard shelled foods like crabs, shrimps and such then they can start to overgrow to the point of being life threatening due to starvation.

Since freeze dried krill is probably not wearing down his teeth , he just might be chewing on the rocks to keep them from becoming a problem...which would be pretty neat if you ask me. :o