The stamps would mainly have been constructed of wood, the actual business part for crushing would have consisted of long poles clad at the end with an heavy iron shoe, this is the part which pounded the ore to the required size.
The poles would have been lifted by cams on a roller driven by a water wheel, the stamps would have fallen on to the ore crushed by their own weight, crushing the ore against almost certainly a substantial mortor stone or in later examples a against a caster iron mortor stone.
Later examples were made almost entirely of metal, allowing greater force to be used, The Blue Hills Stamps are of this later type, but would give a good idea of how a set of stamps works.