Why does my dog eat rocks?

So she is a big eight month old chocolate lab, and she picks up rocks at runs around with them in her mouth. She doesn't usually swallow them but she has once and she was choking on it at night and we woke up and there was a rock in the living room with like slime all around it so i guess she puked it up. when we throw a rock she will go to great lengths to find it, and then she will go out and find it, the same one in 2.5 tons of rock. she chews on them and we are afraid that she is either going to swallow them or wear her teeth down so much that she doesn't have any left. how do we get her to stop? or should we just not care and let her do what she wants?