Dogs often eat things that make us scratch our heads.  But dogs sometimes try to eat inorganic matter that has no digestive upside at all. Rocks provide an example of one of the more common non-food objects dogs chew on. This behavior is sometimes called pica. 

Chewing rocks can be dangerous to more than your dog's teeth or soft mouth tissues. It can lead to intestinal blockage, vomiting, diarrhea, or even choking, if the rock is large enough to block your dog's throat. 

What causes this bizarre behavior? There are a number of possibilities that range from medical to behavioral. Chewing rocks may be one way for an attention-starved dog to get noticed.  It may also be rooted in a medical condition such as a mineral deficiency or diabetes.  If your vet determines that there is no underlying medical cause, devise a strategy to combat the behavior.  Pica is not limited to rocks either.  It can be any item that is non-nutritional like rubber bands, sticks, carpet, etc. 

Pica may be nothing more than an outlet for chewing. One strategy is to keep a good number of chew toys on hand, and rotating them every several days. For instance, put three chew toys out for your dog to play with, and keep three hidden. After several days, bring out the three hidden toys and remove the other three, so they always appear to be "new."  

Examine your schedule as well.  If your dog is spending too much time alone spending more time with them may correct the problem, especially if the time includes plenty of exercise.