No they are not.
They are accepted widely and have been the favourite reserve of the empires that dominated militarily and now economically, but it’s quite arbitrary and for large swaths of history those same empires banned the usage of one or both of those metals by the average merchant in much of their empires.
They, like any other currency, are backed by the Lead Standard (previously the Iron Standard, before that, the Bronze Standard), that being the martial might of those who believe their value.
Silver has significant cultural value and minor industrial value, mostly in it’s resistance to corrosion and anti-bacterial properties.
Gold has more cultural emphasis than most realize and some largely out dated industrial applications, mostly in it’s pliability to conductivity ratios and molecular stability to be worked into extremely thin layers for particle experimentation and light manipulation.
These metals have financial value all out of proportion with their inherent value. Any time this is the case it is because someone of power wants that value maintained.
/End specific response
That said, this one would note that, in this game, it is probably more important to stick to the source material.
Coin of Gold and Silver are mentioned significantly, as are gems and jewels.
Pearls often included in with precious “stones”, especially in pirate hauls, which were a cornerstone of Hyborian Economy, despite lacking a Mediterranean Sea…
If we want to expand value items, how about Silks?
Silk, like Ivory, was an extremely popular trade good.
At least the non-Craftable Black Silk? Especially as it has only one use and that use is… well it’s not useful at all.
If we really want to go historical, how about spices?
~Looks at how easy spices are to acquire in game~
Nevermind