Warning
My lab likes to pick them up and carry them about never seen him eat one but in 2005 i have a £500 vet bill because he had eaten one and it got stuck in his small intestine.
Not much you can do to stop them other then keep taking them off him and saying no?
Sorry i have no better advice other then to watch out, my vet sees alot of dogs that have eaten stones its common prob but just a habbit normally find its nothing to do with food etc just dogs being odd