Boiling can be a dangerous way to sterilize rocks, any air pockets inside the rock can rupture sending rock fragments and boiling water flying. If you raise the temperature slowly it will reduce this risk. Personally I don't boil anything. If these rocks were collected at the lake (freshwater) I would rinse them in hot tap water then mix up a small amount of saltwater and let them soak in that in a bucket for a week or two, if collected at the ocean I would rinse the same way then soak them in tap water for the same length of time. If your really paranoid you can add some bleach to the water and let them soak like that for a couple weeks then rinse again and soak in clean water without bleach for a couple more weeks. Then I would add some water conditioner (chlorine remover) and soak for another week or two. This will neautralize the bleach.