Gold is an accident of history. It's valuable because it's shiny, it was one of the first workable metals (gold jewellery dates back at least to Sumerian times), and it required effort to produce (too Marxian?). Also, royalty and nobles had control of most gold because they controlled the means of production. With the connection to wealth and power, it was then used as a medium of exchange because let's face it, shekels of grain are somewhat inconvenient. Golds value persisted because of its durability, history, beauty, but mostly because it was always in demand.