SOME rocks do, most of what you find in rivers and such should be fine. The rocks that are explode are generally volcanic in nature and have trapped pockets of gas or air in them. As well they are porous and air enters then naturally. It is whent he air gets hot and expands that the rocks can't handle the stress and "explode"
The small smooth river rocks and such you will be picking up should be more than fine to boil. This is why I generally have the lid on the pot I am boiling to help protect the area. I find it works perfectly fine.
Just put enough in at a time to layer the bottom of the pot and put in warm water. Bring the water to a boil with the rocks in it, boil for a about 15 minutes. Then turn it off, take the pot off the heat and let it cool. Pour out most the water and let it finish cooling. Then get something clean you can really scrub the rocks with, do that under hot water, hot enough that it cleans but not hot enough it hurts your hands. Rinse them well and set them aside.
To further clean them you can wipe them down or soak them in some vinager and then once again wash them off in hot water. Then they should be fine to put use.
Remember safety first.