Rocks for Sale

I was in a store Saturday, shopping for some gifts, when I saw a stack of rocks on sale.  It looked not too different from the one on the left in this picture, which I stacked at the beach a week ago Saturday, except that the stack in the store cost about $35.  The stack in the store wasn't quite as big as mine, and was held together by some kind of glue.  So if I go down to the 99 cent store and get some Elmer's....