imo, it hasn't to be gold, it's on the country or group to deside their backing of the currency. The market will make the price. Each country/group can have different "basket" as backing for the currency, that can be % bnp+% silver/gold stocked+% minerals not mind in owned land+etc, etc. A country/group will sometimes change the % of a part in basket (and by that of more to be compensated) by selling buying silver/gold stock or mining minerals for example.