"What medium of exchange will be acceptable to all participants in an economy is not determined arbitrarily. First, the medium of exchange should be durable. In a primitive society of meager wealth, wheat might be sufficiently durable to serve as a medium, since all exchanges would occur only during and immediately after the harvest, leaving no value-surplus to store. But where store-of-value considerations are important, as they are in richer, more civilized societies, the medium of exchange must be a durable commodity, usually a metal. A metal is generally chosen because it is homogeneous and divisible: every unit is the same as every other and it can be blended or formed in any quantity. Precious jewels, for example, are neither homogeneous nor divisible."