While it is not abnormal for many kinds of animals to occassionally eat rocks and dirt, especially when it is around or sticks to legitimate kinds of food, no well-fed creature should be eating an abundance of these materials.
As always with these postings, it is difficult to tell whether the original poster was worried about a couple of rocks one or two times, or a bunch of rocks over many many weeks. If it is the latter case, I stand by my original advice. If it was only a rock or two, who knows? Turtles are turtles.