- Coinage is minted by the Duke and formally called "dragons", featuring the image of a dragon on one side and a profile of the duke himself on the other. The dragon images are different, so for fakery it is not enough to just change the color of the coin, say making coppers look like gold - someone looking closely would see that the dragon was the copper type, not the correct type for gold. It's not fool proof but it does make cheap counterfeiting a step more difficult. The duke currently mints copper, silver, and gold coins. Platinum coins are infrequently minted but have been in the past and could be again in the future. The coins are commonly called "dukes" by the populace (out of earshot of the duke's officials anyway) as in "that will be one silver duke".
- Copper dukes are the standard coinage of the common man. Most transactions not handled by barter will be handled using copper coins - individiual plain or low quality meals cost a few coppers, cheap homespun type clothing is priced in coppers, very basic tools and dishes are priced in coppers. 10 coppers = 1 silver
- Silver is the standard coinage of the merchant, trader, and craftsman. The standard wage for a laborer is a silver a day (roughly a copper per hour). Good quality goods tend to be priced in silver dukes - a good meal at a decent inn, a glass of quality wine, respectable clothing. A typical merchant or guild-rated craftsman will make many daily transactions in silver, using coppers as "change". 10 silvers = 1 gold (=100 coppers)
-Electrum is an old out of date type of coinage that is not commonly found in day to day transactions. Wealthy families may have a reserve of it and it was more common in ancient days and so may be looted from old ruins, but it is not minted by modern rulers and may have limited fungibility in rural areas unless taken to the money changers.
-Gold is the standard coinage of the wealthy, nobles, and the adventuring class. They tend to operate on a different level than day to day workers and crafters and often seek specialized equipment and luxury goods. The finest clothes have a price that begins in gold pieces as do the finest of tools and most gems and jewelry. Good quality weapons and armor also are priced in gold. A night at a fine inn will typically cost 1 gold duke (more for a special room).
-Platinum has had ups and downs as a coin of exchange. It has never replaced gold as the standard upper-end currency, but it is useful for traders seeking to compact the bulk of their wealth when traveling great distances. Beyond the occasional run of platinum dukes, the other form of platinum currency that is relatively common is the trade bar - this is a 10 platinum piece weight of the metal in the form of a small bar instead of a coin with a value of 50 gold dukes. These can be stacked quite solidly in small chests and will not shift or clink as easily as loose coinage. Originally a dwarven innovation (who also make gold and silver versions), they are increasingly popular among human traders but are not typically used in day to day transactions.
- Loose gems and jewelry are another form of transportable wealth associated with the wealthy , but are not as popular with merchants and traders, being seen mainly as luxury goods. Some couriers will carry them covertly to settle long distance deals. They are also highly valued by elves and favored over large masses of metal coinage by that people.
DM Note: I do not usually make a huge deal out of what kind of coins the party finds in a treasure horde but I do like to throw in complications occasionally. It gives the players a little more to do in town if they have to go talk to a moneychanger (also a good source of news and rumors) or if a scholar gets excited about the huge ornate gold coins they pulled out of a dungeon - "Those are Imrryrrian Golden Wheels - where did you get them? That empire fell 1000 years ago!" That kind of thing. They can be an adventure hook, give me a chance to add a little color to the world when I'm in the mood and it doesn't involve a bunch of mechanical complications or new rules that need to be noted down.
I do like to throw in item quality on occasion - if a character is talking to a noble, if their outfit and gear is mostly used, copper or silver level stuff, they will probably have a harder time than if they have gold-level gear and clothing. It's not an iron-clad thing but it can lead to an occasional penalty or bonus - hirelings are impressed by the jewel-encrusted sword and gold-inlaid helmet to a degree, moreso than the blood-spattered chainmail with a few obvious patches on it. Again it's not a huge part of the game but it does help rein in the total cheapskate player and reward the player who pays attention to the details.