Electrum is an alloy of gold and silver. Electrum is naturally occurring and is also produced artificially through metallurgic processes. Etymologically, the word is probably connected with ἦλέκτωρ (the sun), the root meaning "brilliant." In ancient literature, the word also referred to amber. The earliest passage in Greek literature referring to the metal is in Sophocles' tragedy Antigone where mention is made of Indian gold and electrum from Sardis as objects of the highest value. This reference is almost certainly to native electrum deposited by rivers in Asia Minor, particularly the river Pactolus. Pliny states that electrum contains a fifth |part| silver (80% gold) while Isidorus (c. 560-636 A.D.) states that artificial electrum is composed of three parts gold (75% gold) and one |part| of silver.
Electrum hekte, Phokaia, Ionia, c. 387-326 B.C. (Forum Ancient Coins)