The use of coins in commerce began in about the seventh century B.C.E. in Anatolia, modern-day Turkey. The first coins were made of a naturally occurring alloy of gold and silver called electrum, but a differentiation between gold and silver forms of currency quickly developed. In Roman times, an aureus (a gold coin) was worth twenty-five denarii (a denarius was a silver coin). Standardization of the weight and composition of coins made it easier to assign a numerical value to such funds and exercise greater control over commerce. Even though monetary exchange is increasingly virtual, gold is still often considered a solid asset.