@21echoes covers everything else I wold add on this.
The US currency used to be backed by gold up until 1934 [1], as did several other countries[2]. But as times have moved on, there are several other minerals which have become more important, I think it would become too difficult to rely on just a single precious metal these days.
[1] http://www.federalreserve.gov/faqs/currency_12770.htm [2] http://en.wikipedia.org/wiki/Gold_standard
Whenever I come across the argument that we return to a gold-backed economy, this is among one of the first things that comes to mind. Gold is worth so much because it's considered a precious metal and has numerous uses in industry. But besides the extensive history of its worth throughout the ages (which probably maps well to its chemical attributes, such as malleability), there's nothing that suggests it won't eventually become worthless if civilization values something else (salt, platinum, clean drinking water).
I used to think I was a little crazy whenever I'd have such an argument with my inner monologue, but now that I see the same point raised by others, I'm not so sure.
Maybe I still am crazy.
Weimar, Austria, Zimbabwe, Argentina... It's a long list of 20th Century, central bank, paper induced economic disasters: http://en.wikipedia.org/wiki/Hyperinflation
Can you find any hard-money regimes like that?
Did it never strike you as odd that when the robber barrons were deposed by the class of disenfranchised oligarchs they were awarded the Federal Reserve as compensation.. Yes they were quite pleased.