However, even if only gold coins were in circulation in any given state or kingdom or forum, the sovereign or ruler of such is who says and thusly it is that authority that makes it money. If I created crystal containers and stamped $20 on them and filled them with pure water and decreed them to be currency in the Land of Me, sure sure the water inside the crystal container has a value itself --BUT THAT IS BESIDES THE FUNDAMENTAL POINT-- being that it is *I* that determines that water contained in such and such a container stamped in such and such a manner to be worth $20 and is good for currency. A state/kingdom/city-state doesn't have an obligation to make gold or silver its currency although it might have to make sure the system is overall just and balanced.
The perceived value of gold or diamonds is 100% separate from the authority or sanction backing its use as a currency (i.e. melt-value is separate from declared value). Clearly gold nuggets and gold bullion isn't coin--it should be easy to comprehend. In the United States it is silver in a certain form that is a dollar per the Coinage Act of 1792.
Per the Coinage Act of 1792, a dollar is coin containing silver and the Gold Eagle coin's value is defined in terms of the dollar not the other way around. The lawful dollar was not originally defined in terms of gold.
See also: pound sterling and Spanish dollar.
An ounce of gold **is not money* although it may be valued in terms of money. Money has the backing of a sovereign or ruler of its associated forum.
That ain't money.
That is a photo of official coin/currency of the United States--i.e. money. Certified, stamped and underwritten. The industrial value is separate from its decreed value.