Another newbie question...

I am in processing of collecting decor for my tank. The only free rocks (and I need free right now) I have available to me are granite. They look decent, but I am concerned about the weight. They are not boulders, but I could end up with 20 - 30# of rocks easily, since granite isn't exactly light. So, my question is: Since the tank frame lifts the bottom glass up from touching my stand's top surface, isn't it possible that the rocks could crack the bottom glass? Should I try to find some sort of rubber mat or something and cut it to fit just under the tank?

My tank is only 36x15x16 and the glass is 1/4" thick, as far as I can tell.

Thank you for your time...