IMO, try to avoid placing rocks directly on the glass, especially if the bottom is tempered glass.
I use lighting diffuser personally. When placing the rock, especially larger ones, make sure they are well seated through the substrate.
I'd recommend putting it on the glass. If it has sharp points, you could put slate underneath it as well to prevent a blowout.
I like that idea of using slate, I also read or heard of people using foam?
?
Foam would work assuming it was non-toxic, but pieces might break off over time and float about the tank. More likely you've heard about people putting foam under their tank, between the tank and the stand, to help it self-level slightly and remove any imperfections in the stands surface.