A sovereign or quasi-sovereign can declare gold and silver fluffy pink bunnies. That won't stop the rules and laws of nature and supply and demand from working.
Gold and silver are commodities regardless of who may decree whatever. This refers to industrial or commercial usages of gold and silver (aside from these possibly being a medium of exchange.
When the decrees don't work, expect restrictions or confiscations to occur. Restrictions and confiscations are attempts at price control ....
Gresham's Law can be quite unforgiving ....