You don't need to boil rocks. You risk injury to yourself as they may shatter. Just give 'em a good scrubbing and soak them. Drain and soak. repeat until the water is clear. give 'em a scrub with a mild bleach solution if you're worried about anything alive on them. then soak and drain a few times.
Q
thank you! boiling is not a cure-all.
if there is a quarry or mining operation nearby, you should be able to get some for cheap, as well.