if its rocks that people actually got out of the river then what i did was boil it and some people say to use vinegar and if it bubbles dont use it. i just boiled mine for a good hour and have never had a problem, i got mine from my girlfriend's lake house on the tennessee river.
i soak mine for at least a week, usually two.
I'll put a little bleach in the first cycle, then just keep soaking in water, changing the water out everyday.