In the mid-6th century BC the Lydian King Croesus (c. 560-546 BC) introduced a momentous innovation: the bimetallic coin system, the circulation of gold and silver coins at the same time. Before that coins from electrum had been in use, a natural alloy of silver and gold.
One reason for this change supposedly was that the ratio of silver and gold in electrum is not constant. The proportion of gold in electrum remains uncertain and can only be guessed by the coin's color. The value of a silver and a gold coin is fixed, and at the most subjected to the fluctuation of the bullion market.