One gold coin = 10 silver coins = 100 copper coins. A platinum coin is worth 5 gold coins, but platinum coins are very rare. Most platinum mined by mortals is now forged into high end jewelry. Most country's mints will ridge the edge of their coins to prevent coin shaving. Some mints are operated by agents of the local ruler, others by the Masks, more than a few by both. Either way they use the same standard weights for coins. The standard weight of all gold coins was set at one fiftieth of a pound. When gold, silver, or copper is transported in in the form of bullion, it's typically stamped with it's weight and equivalent value. Five pounds of a precious metal is worth 250 of the coins of the same type. The standard system of measures was adopted by most of the human nations in the Third Age. Most nations prefer to mint their own coins, commonly putting the face of the ruler on one side and the national coat of arms on the other, but all nations use the standard sizes so most merchants don't care are which royal face is on the coins.
Phidas gave mortals the concept of currency as his Gift to mortals. During the First Age, most currencies were based on precious metals, specifically. Gold, silver, and Platinum. Currency denominations and forms varied a lot from nation to nation. During the Second Age, the Keepers and the Masks worked together to create a standard system of weights and measures to standardized coinage. By the end of the Second Age, nearly every elf nations had adopted the standard exchange rate. Early in the Third Age, human tribes relied on barter or widely varying local currencies but most humans societies adopted the standard coin system before the end of the Red Era.