It is hollowed out so the rock hammer would fit inside it. Andy presumably hid the rock hammer in the Bible. The last place Warden Norton would look. It proved successful and Andy managed to hide his rock hammer for nineteen years, and escape by tunneling through the wall. The night Andy escaped he replaced Warden Norton's account book with the hollowed out Bible and leaves a note that causes the warden to drop the Bible in surprise once he realizes what Andy had done.