My 2-year-old son is eating small black stones when he is playing outside. We were really shocked he considered those as `Chips` and he try to bite that stones and swallowing it very often. Now we are not allowing him to play outside because we were really scared whether it will hurt his stomach. 

Could any one please let us know how to stop this habit. How to make him understand `stone` is different from what he is thinking and not allowed to eat.