My 2 cents.
It depends on the density of the rock. I don’t know which rock is denser than an other, but I know you could get one that is the size of a softball and it weigh 5 pounds, and get a rock of a different variety of the same size and it only weigh a pound. And it all depends on how you want to pack it into your tank. I have 45-50 pounds or LR from SWF (Fuji) and it is stacked on a few pieces of lace rock. I have it stacked half way up the tank, and it is not touching and of my glass.
Hope this helps.