I agree with the previous post, wouldn't put the rocks directly on the glass bottom . . . I'd also be careful about placement, I've heard horror stories about rocks moving and cracking the glass . . .
Tempered glass is very strong and can support alot. However if the weight is focused on a single point it can damage the glass. If you are putting alot of rock, one trick you can do is use egg crate between the rocks and glass for added protection.