The coin system does not rest on the value of one material, like gold or a gemstone, since their value can vary a lot and is difficult to estimate. Indeed, the value of a
material depends on their magical content. To have a coin system, the magic inside each coin of the same value needs to be both identical and impossible to counterfeit.
To solve this problem, coins are minted from a cheap metal with uninteresting magic, and they are then treated with the magic of the ruler of the Estate where they are being made. So long as the magic is intact inside the coin, their value is guaranteed by the ruler of the Estate.
Thus, after being minted, boxes of coins are brought in front of the ruler and placed inside special caskets covered in runes. The ruler needs to touch the casket with both hands and push their magic inside. The runes will then guide it so that it goes inside each coin in a specific amount and in a given pattern. The process takes about one hour for one casket and does not require a lot of focus on the part of the ruler.
The design of each coin varies depending on the ruler's desire. Most have the coat of arms of their Houses minted on the coin rather than a symbol attached to their individual person, as that could create the belief that the coin would become worthless after their death. To indicate differences in value between the coins, different sizes and indents on the side of the coins are used, as well as different metal alloys to give them different colours.
I really appreciate the detail in integrating the magic into the coins. It sounds like an interesting event each time they bring chests of coins.
Thanks :D From the POV of the ruler, it's mostly a very boring task they've got to do once in a while XD