Not rational? Gold has real world use. It is in the computer you are typing on currently. As other have noted it's value in terms of trade comes from it being limited, "beautiful," not susceptible to corrosion. Silver shares many of these qualities thus is also popular as a store of value and medium of exchange.
It is a light weight medium of exchange that doesn't have you lugging around 10 gallons of milk that will go bad just so you can buy a side of pork.
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