What distinguishes coinage from all the other things known to me that have performed money functions is that on the one hand, unlike most such things, it bears a sign, and on the other hand, unlike all other such things that do bear a sign (such as paper), it is also inherently valuable.
Seaford advances the definitive structural thesis: the coin is unique in world monetary history precisely because it combines intrinsic material value with a semiotic guarantee, a duality that generates its peculiar psychological and philosophical power.
, Money and the Early Greek Mind: Homer, Philosophy, Tragedy, 2004thesis