Electrum is a not-as-beautiful-as-one-might-think alloy of silver and gold. It was listed as an optional coin in the original boxed and, depending on referee choice, was suggested to be worth either half or twice as much as gold (2 electrum to one gold coin, or 2 gold coins to 1 electrum piece). Besides being a rather ugly metal (see graphic below), electrum coins are significant for another reason.
Ever wonder why Experience Points were abbreviated XP rather than EP? According to at least one person around back on those early days, it was because of electrum pieces (EP).
And now you know!