During their campaign of conquests the Melniboneans stole every bit of precious metal they could find. Coins, art, jewelry etc.
Since then precious metals are no longer used for currency. Common law in human kingdoms now says it is illegal to possess gems, gold, silver or platinum in any significant quantity… less the Melniboneans come looking for it.
That is not to say that precious metals are no longer mined, minted or stored in a treasury. Such practices are still commonplace but precious metals are not allowed to circulate in trade. Instead they are stockpiled and used to tithe the Melniboneans to keep the peace.
Bronze and Steel coins are now the standard currency used for trade. There are 12.5 Bronze coins to a gold piece and/or two steel coins. Rectangular bars are used when coins are too bulky to transport. Bars come in two thicknesses, one equivalent to 50 coins, and another to 100 coins. Bronze bars are smaller in dimension and weight because the coins themselves are smaller.
Even so, without gold, hard currency is too bulky to be practical in trade. The Melniboneans created a banking system where hard currency can be exchanged for a bank writ. In many ways writs are safer to use then hard currency and certainly easier to carry.