I just remembered that the doctor said that MY kind of stones were oxilate based (I was able to take a stone in for analysis several years ago).  I also seem to recall that he had mentioned that some types are due to some sort of "imbalance", and that if that had been the case, some sort of medication would have been needed (hormones?!).  If your daughter does pass gravel, you can get a sort of "filter" that fits over the toilet seat, and will catch it.  If you can take this gravel in for analysis, it helps alot in finding out the reason and treatment.