Originally Posted by DeathBrewer
My friend talked about the exact rocks you're supposed to use. If I ever remember to talk to him about it, I'll post.
Awesome! Not only does it require a correct type of rock, I have to be able to find it!
Graywacke is the traditionally used rock from a historical perspective, but the requirements are:
(1) can take the thermal shock of being heated and cooled
(2) will not interact chemically with the wort itself.
Some rocks will change the pH quite a bit, along with rub off in the beer.