I have no experience with the stone of which you are speaking, but i do have experience buying rock and stone in general, and I can tell you that rocks are heavy. Much more so than one would imagine. Of course it depends on the type of rock...could you ask them on average, how many pieces of what size, will be included in a typical 5 lb package? This could help you estimate better. I know this is pretty general advice (i'm sure you know rocks are heavy!!), so best of luck!!