I think they just like playing with the rocks.. They don't do it often.. But then you mention minerals.. Maybe they need more minerals in their diet.. I feed them Purina Bentiful Original with Beef.. the ingredients seem to cover everything from protein to minerals.. hmmmmm

I used to buy Iams dog food till I learned they are very cruel to the dogs they use for testing the food.. I did a research paper on animal testing and it made me cry. Soooooo sad what those animals go through... Prob Purina is as guilty.. I just have not found out yet...