Boil? For about how long
I am no Rock expert but DO NOT Boil rocks!
there could be a gas buildup inside that when boiled causes the rock to explode.
Now, from what I have read on here, you can pour boiling water over the rocks. You can brush them off with a clean brush, you can pout vinegar or Peroxide (that last might not be correct) and see if it bubbles to know if it is inert or fine for tanks.