Money Makes the World Go 'Round

Since my characters are going to be travelling to various nations, I did some research on coins and currency. Coinage is one of those things that seems really minor, but it's the minor details that more verisimilitude. There's an amazing variance in the kinds of coinage used throughout history. The first coins were created in the 7th and 6th centuries BCE in Greece, India, and China (aka, the earliest civilizations). China even had paper money about a thousand years ago.

Coins can take a surprising number of forms. The most ubiquitous is, of course, the round coin, but we also have square (in modern times, they have rounded edges for vending machines), 8/10/12-sided, and oval (common in the Orient). The edges can be milled, scalloped, or notched, to prevent clipping. Oriental coins often have holes or squares punched out of the middle to permit them to be strung on a lanyard. Coins are always stamped with some sort of design - often it's the likeness of the ruler who commissioned them, or a god/goddess; other common themes are animals, landmarks, or simple designs, often with a legend (writing, often a phrase of some sort).

90% of coins you seen in fantasy novels are copper, silver, gold, and (rarely) platinum. While these were in use in the real world, there were also a variety of other metals used: electrum (a naturally-occurring silver/gold alloy; rare), cupronickel (a copper/nickel alloy, sometimes used to debase silver coins), iron, lead, nickel (used since ancient times), brass (the Romans), and bronze (not that common, surprisingly). Your world could have other materials that are more or less common or valuable. Just keep in mind that we used softer metals for a reason - they're easier to stamp. It just so happens that many of them were also precious metals.

Cultures don't always rely on coinage as currency; many use livestock (chickens, goats, horses, cows), random silver items, https://en.wikipedia.org/wiki/Hacksilver, https://en.wikipedia.org/wiki/Knife_money knives (ancient China), food (salt, cocoa beans, or cheese), or other odd items. Cultures for whom metallurgy is impractical or impossible (undersea races, or those who live in a metal-poor world like Dark Sun) could use tokens of ceramic or bone, or beads of precious/semi-precious gemstones (agate, garnet, jade, pearls, coral, etc.), which could be pierced so as to be strung on a line.

And finally... denominations. Most nations have a base amount (like the Dollar or Euro), which they can then subdivide into smaller units (as coinage) and multiply into larger units (as larger bills or, rarely, also coinage). Coinage typically appears as one or more of the following:

1/100, 1/20, 1/10, 1/5, 1/4, 1/2.

So, for example, you could have the gold Mark as the base currency, then copper (at 1/100 mark), bronze (1/10), silver (1/2), etc. Or you could switch it up and have 20 coppers to the bronze, 15 bronze to the silver, and 3 silver to the gold. It's all up to you.

* Wikipedia entry on coins: A central page where you can check out all the currencies of the world.
* 10 Strange Forms of Ancient Currency: Not all currency was coinage, or even money.