Green gold, also known as Electrum, is a naturally occurring alloy, made up of mainly gold and silver, with small traces of copper that can also be produced artificially. The colours range from pale to bright yellow depending on the proportions of gold and silver. Electrum was used in ancient Egypt, sometimes as an exterior coating to pyramids and obelisks. It was also used in the making of ancient drinking vessels and the first metal coins ever made were of electrum. The Nobel Prize has also been made of gold-plated green gold.
Gold-silver alloy nanoparticles, due to their composition-controlled colour, can be used as contrast agents for imaging of cells.