You've got to be careful. First of all, if it's limestone or other calciferous rock, it will leach into the water and create problems. Second, some rocks have trace amounts of metal in them. Iron would be fine. Copper would be disastrous. Third, if the rock is near a place where chemicals like weed killer or insecticide have been used, or is in a polluted area (like in a spot where a lot of broken-down cars have been parked over the years, for example), it is likely carrying some of those toxins.
For the first, you can do a vinegar or hydrochloric acid test to figure out if that'll be a problem. For the second, you've got to identify and learn about the rock. For the third, knowing about where they come from is necessary.
As long as you scrub and boil them well to get rid of any nasties that might be on them. After boiling let them dry naturally to keep them from exploding.
You might want to drop a little vinegar on them also. If it bubbles it is a calcareous rock and May raise your pH eventually.
i would boil them, but let them cool off before taking them out of the water, they can do a mini explosion if cooled to quick. I would then fill up a bucket pf water and make a not of what the water parameters are. Add the rock and let it sit for a week, and recheck your parameters to make sure there are no major changes. certain rocks can raise your ph.
Yes! Make sure you boil them to kill any bad bacteria, Some rocks can change the water quality so a good test to see if it will change the pH, add a teaspoon of Vinegar to the rock, if it fizzes and froths slightly then DO NOT add it to your aquarium.