There's an excellent NPR podcast about economics called Planet Money. They
did a great show on why gold is the de-facto universal currency.
They break down the requirements and needs of a currency and, accounting for
stability, toxicity, availability, portability, and overall ease of
collection and processing - you are left off with only a few available
metals. Gold is one of them, along with silver, palladium, and the other
metals usually used as a store of value, coincidentally enough.