I think when they're bored and their chewing habits aren't directed to things they can chew on, they'll chew anything including metal, rocks, etc. When I'm training a puppy at home, my husband I keep a small rawhide knuckle bone in our pocket, so it's handy. As soon as we catch them chewing on something bad, we calmly but firmly say "No", and then exchange it with the knuckle bone. When they lose interest in the bone and leave it on the floor, we pick it up, pocket it and start over until training is complete.