Someone else mentioned making gold coins in the future, and a thought struck me; at the moment, coins are only worth the base metal's value. That metal's value is determined by market demand of that metal, and it's supply. That demand is determined by the utility of the metal to the average hearthlander, and the supply.
Now, let's say that gold was added to the game as a minable metal, and only appeared very rarely and was very difficult to extract (Though gold almost never alloys and is nearly always found "native" this is true to an extent- in order to find these native flakes, one must sift a great deal of ore and use huge amounts of water or even quicksilver to purify it. The Gold county up here is richer in split mercury than gold, now). Thus, the gold has a low supply. I still suggest that after everybody has one or two gold coins for the novelty of it, it would become rapidly useless unless a material use for it existed. Yes, it could be made into jewelry, but there's only so many slots to fill with rings.
What becomes obvious is that there must be a gold-consuming use for the material. This does not need to be common, just enough to balance the rate of mining- or even overbalance it, so that there is always a scarcity. These uses would further need to be related to the core gameplay elements and have an advantage that would be as universal as possible, so as to give many people reason to stockpile it the gold for their own or other's use. It would not need to actually occur very often- in fact, it would be best if it took a long time and a significant amount of gold before it could be used. This would lead to large stockpiles of gold, leading to towns having a body of wealth that they can occasionally draw from when in more urgent need, and that thieves may come to steal.
This applies to many other metals, of course. So, without further ado, my personal metal ideas.
Gold: Jewelry and furniture. Perhaps some high-level item for towns, such as a throne, would require gold leaf for gilding. Also, perhaps a mystical use. Since gold represents wealth, nobility, and prosperity, maybe some ritual done by the chieftain would bring health and happiness to all the villagers around him. The gold would be gone, save for a warm sunny glow around the gilt individuals. Perhaps this improvement would be permanent, until the hearthlander died, or perhaps not.
Silver: A more common jewelry metal, and suitable for some tools and even weapons (though more likely as a surfacing material than a main metal), making silver arrowheads a possible expenditure of the metal. Silver has long represented wisdom (at least in a large portion of the western world), and it would be reasonable for a lawspeaker to perform a ritual with silver implements to bring great learning to the villagers affected, increasing LP per task.
Copper: A very mutable metal, which forms many different kinds of bronzes, brass, and other useful alloys. It's basic use would be as the root of a tree of alloys, but it would probably be far too small a value to mint a coin. Copper has been used for jewelry, and utensils. It's just much better used to make alloys. Copper has been called the Human Metal, as it's pretty much always been useful. And yes, hypothetical firearms would need copper cartridges, I'm sure.
Bronze: I would hope to see bronze be the industrial cheese- a tree of possibilities with many different useful branches. Some bronze weapons can be nearly as good as steel, and easier to make. Brass could almost be included in the bronze tree as a more decorative version.
Iron: Current cast iron and wrought have their uses, for specific items and as steps up the steel ladder. I'd like to see the current steel be a low-grade steel that is fine for basic work, but to create the absolute best weapons and tools, one must fold the steel the hundreds and hundreds of times that is traditional of great blades, taking hours of laborious work to produce truly great things.
Tin, Zinc, Lead: Mined as part of the copper/bronze tree, these metals would also be so purely economic or industrial that they would not have much coinage value. They would make excellent metals to be localized, though. I imagine a village in Tin country trading with the southern lead mines, and so on. Other potential uses are as ingredients for pigments.