You need to test the rocks. The easiest way this can be done is with vinegar. You pour it on the rock, and if it bubbles the rock is not safe to use in your tank. However if it doesn't bubble, it may still not be safe because vinegar is a very weak acid, and it may have just been too weak to cause a reaction. In this case try using a strong acid, like muriatic acid. If it doesn't bubble with this then you should be perfectly fine.
Also soak it in a very weak bleach solution. Like 2 gals to 4 tbsp bleach. And then rinse, rinse, rinse, rinse, rinse! And when you think they are clean, rinse MORE!