If youre serious, youd need to make sure thw rocks you choose do not have any sulfurin them (and thats damned difficult because defect Xl lattices usually contain elements of the Sulfur family of elements).
Hyperthermophylic archaea can live in boiling water as long s they can hqve some sulfur to munch on.
A rock Id choose wouldnt be a rock, itd be a peice of 1,1silane in a ring with (cyclo) siloxane (hydrated forms of quartz gel mixture made in a lab and can be made to "set up" like a paperweight.
OR have a pice of crystal glass drop forged with a hole so that you can attacch the implement to be sterilized. I think the silane cyclosiloxane has a p grav about that of quartz so Itll sink
I dont know why you wish to do this when pressure & T works fine.
what are you sterilizing anyway?