The type of rock doesnt really matter as much, as making a lot of good hiding spots and caves for them to retreat to.
My tank is Labs/Dems and I have all diff kinds of rocks in mine and they are constantly breeding and seem to thrive very well. Just pile them up so there is plenty of spots for everyone and at least half way up the tank and they should be fine.
If there is a specific rock that they like more I don't know. Maybe if you have soft or unbuffered water a type of limestone or something similar would help to buffer it.