I think this shows the problem of non fiscal pegs - the value of rare earths or duterium is going to be impacted by technical change. For example let's say that a new processing technique means that a rare earth can be extracted at 20% of the current cost, or a new deposit is discovered. The whole economy is now flexed around this change... which has nothing to do with the economy.

I don't think that there is a fixed asset that could be suitable as a proper peg.