https://en.m.wikipedia.org/wiki/Electrum
Naturally occuring gold/silver alloy. Used for the earliest known coins, the caps of Egyptian pyramids, etc.
Or, in the current context:
https://www.dndbeyond.com/sources/basic-rules/equipment#StandardExchangeRates
Coins worth 5 silver pieces, or half a gold piece.
In my setting, I just use them like a $50 bill.
I mean for rarity value alone they gotta be worth 2, 3 gold each
I like to use them as part of treasure hoards for creatures that are either primitive or ancient, as a nod to the difficulty in separating the alloy. For the latter, sometimes the coins have value as archaeologic artifacts moderately higher than their own value, if the party invests their time and ration resources to find someone to pay that much.
I played with a DM who only used them in elvish cities, no one else would trade in them. That is now in my world too.
Smug elves and their special currency.



