My understanding is that the coin's value is derived from it's weight and the precious metal it is. So when you get e.g. A Denarius, it's value was in the weigh

My understanding is that the coin's value is derived from it's weight and the precious metal it is. So when you get e.g. A Denarius, it's value was in the weight of silver it is. The coin was a way to folks to know that it was a specific weight, and therefore worth a certain amount of currency. I think a much more common thing was to shave off part of the precious metal and keep it (since the metal is worth money).