In this era, the currencies that are circulating in the Mediterranean trade were gold and silver. The Persians mainly use gold coins, while the Greek city-states did not have so many gold mines, so they mainly use silver coins. Because each city-states had the right to cast their own silver coins, the whole Mediterranean currency market was quite chaotic, and the weight of the coins issued by different city-states and the purity were not uniform, which brought difficulties in trading, hence why currency exchange came into being.