I'm always really skeptical about stones like these, I hear too many stories about water chemistry disasters after trying out rocks that weren't necessarily meant for aquariums.
Boiling or pouring hot water will only sterlize or kill the germs, but this will not change the ability of the stone to slowly release sediments or elements over time that might change the water chemistry.
It's fine to be sceptical, but the vast majority of rock won't alter the water at all. Usually it's pretty easy to tell if they will even without trying them out in a tank.
You're pretty safe as long a you aren't using limestone and some sandstone etc. You can even use these depending on your fish etc.