While people had used gold and silver bars for trade for centuries, the Kingdom of Lydia was the first to issue standardized coins. These first coins were made of electrum, a naturally occurring alloy of gold and silver found in the Pactolus River. Each coin was stamped with an official seal (often a lion) to guarantee its weight and purity. This innovation revolutionized trade by eliminating the need to weigh and test metal during every transaction, allowing for much faster and more reliable commerce. The wealth generated by this system made the Lydian kings, particularly Croesus, legendary for their riches, leading to the expression 'rich as Croesus.' The concept spread rapidly to Greece and beyond, forming the basis of the modern global economy.