It doesn’t have to be physical gold. 100 years ago when most of the world was on the gold standard, paper money was exchangable for gold. Right the way up until 1914, you could go to the Bank of England, hand in a £1 note and get a 22 carat gold sovereign in return. It was a system that worked for over a century before it was usurped in favour of fiat currency which is based on nothing more than what the bankers and financiers say it is worth.
The problem IMHO is that except in very small local systems, even a economy based on gold coins is not beyond government manipulation, and hence not “honest” in the sense we mean here. A certificate backed by gold held in reserve is only “honest” if the number of certificates issued is limited by the amount of gold being held (obviously). I suspect that if everyone 100 years ago went to the bank on the same day to exchange their certificates for gold, the truth is that there would not be enough gold to go around even then. Had that actually happened, or even come close to happening, the government would have no choice but step in.
But would you trust ISIS paper anywhere outside of Beheadistan? For that matter, would you take ISIS paper IN Beheadistan without an executioner standing there to close the deal?