Generally, rocks/stones that do not fizz when vinegar is put on them, have no sharp edges, and are not VERY heavy, (by this I mean a rock that seems heavier then it should be for it's size), are okay for use in tanks if they are well cleaned.
Rocks which fizz contain calcareous minerals which will harden your water and raise it's pH, (although this is a good thing in a rift valley tank).
Sharp objects, well that is obvious.
Overly heavy materials probably contain ore minerals which can leach metals into the tank. Now some of these are harmless, but unless you know exactly what it is, it is best to avoid them.