I have heard the argument, repeatedly, that because D&D uses the gold standard it is somehow not as realistic or representative of the middle ages as certain other RPGs that I could name (Hârn, for one). While it is true that D&D generally does not represent the middle ages very well, I would put forth the argument that this is not because of the gold standard in coinage. Indeed, I have tried to align the 10th Age far more with the historical middle ages myself, and yet I never changed the coinage from gold to silver. For a number of reasons, I don't believe that silver coinage represents anything fundamentally true about the medieval period, and I shall enumerate that belief here.
The first step to determining whether or not silver is a necessary requirement when representing the middle ages is inquiring why there was, historically, a use of silver coinage in that period. The reasons for this are manifold and to understand them we have to go back to the late classical period and examine what exactly happened to the gold coins of the late Roman empire.
Throughout most of Rome's lifespan as an empire, a complex tax structure enabled goods to move to and fro throughout the length and breadth of its domain. The treasury paid merchant interests to ship grain from Egypt but also to carry troops here and there and to carry tax farmers. There were two important spines of the tax structure: north-south, from Rome to Egypt, and east-west, from Rome to the Levant. Both of these spines ended in wealthy regions where gold mines pulled a lot of that substance from the ground.
Because of the subsidized shipping, the senate could afford to pack the ships they were already hiring with gold from the mines in North Africa and the Middle East. They carried that gold back to Constantinople, Rome, and Gaul were it was minted into coins for circulation amongst the populace. The easy availability of gold in North Africa, the presence of the tax-spine which made its transport so cheap, and the lack of easily mined gold in most of Europe, meant that while the gold was being imported from other more gold-rich places, the European provinces would never have to attempt to find sources of their own.
Then came the so-called "collapse" of the 4th and 5th centuries. Rome went to war with her own armies and mercenary leaders, inciting the Romanized general Alaric to march on the capit mundi and threaten to sack it. He eventually, against his own wishes, did sack Rome when the senate refused to pay him and his men the back pay they were owed.
The Gothic Wars ripped Italy apart as the emperor in the east attempted to knit together the dying fragments of his western empire. The wars and bloodshed of this period precluded the peaceful travel across the Mediterranean. The east-west tax-spine was shattered by Persian attacks on the borders of the eastern empire, and the north-south spine by the fighting in Italy. North Africa was suddenly taken away from imperial control by a breakaway group of rebel tribesmen called the Vandals.
Thus, gold went out of Europe's reach for the next several centuries. Byzantium still had recourse to golden coins (which is why golden coins of all stripes were called bezants in the middle ages), but Europe had no way of acquiring it until the new gold mines of the Renaissance in the eastern portions of Europe began to pull up enough new gold for Florence and Venice to mint gold coins.
Even more so, the economy suffered a terrible blow as a result of the fighting across Europe and the population suffered likewise. Trade crumbled, the roads were not safe, and mercantile enterprise (as paid for by subsidized imperial ships) ground to a halt. Some little regional trade still functioned, and even some international trade with India, but the methods had changed drastically. There were no ultra-wealthy senatorial families that owned farms in North Africa and Gaul, there were no powerful merchants who could sell pottery from a factory in Italy to the farthest reaches of Britain. Large-scale trade was almost obliterated.
This obviated the need for a high value coinage, as the trade receipts simply didn't support the amount of money they had a century earlier. Europe could not get to gold but Europe also did not need gold.
What about this is distinctly medieval? The fact that it was post-Roman, perhaps. But I have several arguments as to why this need not be the case in a fantasy setting and that calling for a silver standard for fantasy settings is not necessarily any more sensible than a gold standard.
- Not every fantasy setting has a Rome analog. Without the collapse of the Roman tax system, the economy would never have failed as badly as it did. It is certainly possible for an alternative form of mercantilism to have developed in its absence, one which runs on gold.
- Not every fantasy world has the same geography as Europe. Gold in Europe was hard to get out of the ground. It was much simpler to transport it from North Africa and the Middle East. In any given fantasy setting, this might not be true. Gold mines might be more abundant!
- Dwarves. The simplest and fastest answer as to why gold is a valuable trading material and gold coins are so numerous in the 10th Age is because dwarves mine it. They have mining techniques far in advance of anything our ancestors possessed during that period.