Why would a species (lets say badgers) evolve to eat rocks?
Evolution works by gradual change over large periods of time. The only real reason for an animal to change its diet is if it becomes unavailable for some reason, and they cannot move to find more. If the badger's food source disappeared they would just all die. If it slowly went away over time, and they couldn't move, they would adapt to eat other, similar stuff. Rocks don't have anything in them to fuel animals metabolisms, so even if they could chew the rock, they wouldn't do any good.