sometimes boiling can cause the rock to explode or break...
... letting soak in hot water will also do.
...first test to see if teh rock would affect teh pH. do this by adding vinigar on the rock. If it fizzes tehn forget it.
after the rocks has been soaked and and scrubbed clean then it should be ready for the tank.
Keep a close eye on the water conditions and the fishes health for a little while. If nothing changes then you are set. Be ready to do ample water changes if not
Note that certain types of rock will alter your water perimeters!
For example, chalky rocks, like coral and limestone cause the tank water turn hard (higher pH), which can be bad for S. American freshwater fish (piranha's included!), because the water in their natural habitat is soft and acidic.
prior to boiling you should check for a few things, the pH as stated above, and whether it will effect your general water hardness in your tank, you would be suprised at how much a rock could change that. You can test that by placing the rock in a bucket for a few days and monitoring the hardness, or read up on what rocks will leach minerals into water, mostly rocks that are comprised of many minerals are the bad ones, I just cant remember the scientific name for them right now