Hadfael, you are an idiot.
Is the money in your pocket made of the gold it is based on? When you invest in the stock markets, do you have an actual chunk of the concrete the company is made out of? No. Learn about economics before you toss your inane dribble at me. What an economy is BASED on does not mean that is what the money is MADE out of. If your economy was based on bananas, you would not hand out bananas at the market like some fool. You wold have coins that were worth so many bananas. Does the original banana last? No, but you can exchange your coin for an equal banana later. In some games and books I have seen, water is the base of the economy, and coins were worth so many barrels a peice. Why? Because water was rare.
Anything can be used as a base for money. Land, food, salt, gems, water, materials, trees, shells, oils, or even services. Our own world has seen all of these as the base of money. Gold is just what most of us use now.
*edit* Look at the movie Waterworld. The three most common and cheap materials we have nowdays are what the economy is based on. Water, dirt, and paper.