Electrum

Electrum is a naturally occurring alloy of gold and silver, with trace amounts of copper and other metals. It is found in rivers. 
The ancient Greeks called it "gold" or "white gold", as opposed to "refined gold". Its color ranges from pale to bright yellow, depending on the proportions of gold and silver. It has been produced artificially, and is also known as "green gold".
Electrum has been used for coins.