Gold and silver became money for similar reasons (probably). Kings and Emperors found that supplying their armies was much easier (especially in peace time) if they used the method of taxing the population in gold and/or silver and then using that to pay their soldiers who would supply themselves on the open market. That way you did not have the standard problems of a planned economy. The peasants have now need for gold, so they would not need/want to trade stuff for it unless they were forced to have some to give to the tax man. It's much more complicated than that, of course, but like fiat money, gold and silver do not have much intrinsic value besides using it to show off your wealth.