i have 100 lbs of rock and 150 lbs of sand in my 55 gallon (standard dimensions) glass tank. i think you will have a hard time breaking the bottom glass of the tank with rocks that are simply resting on the glass... dropping them, thats another story.
if you are really concerned, remember that flat bottoms will spread the load, and you will not have any concentrated forces on a small area of glass. concentrated loads are the only thing that i can think of that would result in a failure of the bottom glass...
~mike