electrum

hopeapitoinen kulta, elektrum puhekieltä amber Amber.
An alloy of gold and silver, used by the ancients; now specifically a natural alloy with between 20 and 50 per cent silver.
1995, Paul T. Craddock, Early Metal Mining and Production, page 111:

Native gold almost always contains silver in amounts varying widely between 5 and 50 per cent. This natural alloy is known as electrum although in classical antiquity where the word originated it seems to have been used for an artificial alloy of the two metals.

A natural alloy containing more than 20 per cent silver is called electrum, and was regarded by the ancients as a different metal from gold.