border collies are often quite mad. don't leave it outside unattended, give more attention to the dog, border collies are meant to be kept active, they are not meant to be house pets, they are bred as workers.
birds eat stones to grind the grains they eat.
dogs eat grass when they have parasites.
i realy don't know why your dog eats stones, but could it be parasites? try asking your vet ;) (and let us know the answer)