rare Earths have a pretty big downside of having huge swings in the supply when there happens to be a discovery. you want a pretty stable supply for a reserve currency / backing commodity.
oil is fairly difficult / expensive to transport and store, certainly at volume.
With the current communication capacity, "transport" isn't horribly relevant. Free exchange of any currency backing substance would be as bad as bitcoin for fraud. So why bother moving it? Oil changes owners all the time without going anywhere.
oil is fairly difficult / expensive to transport and store, certainly at volume.
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