3: The Rent of Mines
The rent of mines is of particular interest, as they are similar in some ways to agricultural land and different in others.
As with farmland, mines yield a product from the land by application of labor. Also, rent paid on mines is driven by the value of their product. Also, the most fertile mines are first worked before miners seek less productive ones. Improvements can be made in mining technology to extract more metal with less labor. Et cetera.
One key difference is that technology can increase the efficiency of labor - to require less effort to extract more metal of a mine - but there is no technology that will make the mine, itself, more fecund: there is a finite amount of ore in any mine that, once carried away, is forever gone. One can plant more corn in a field or fertilize a field to produce more corn, but cannot plant more silver in a mine, or cause the mine to produce any more silver than is existent there in the first place.
On the topic of precious metals, the value of metals in Europe had long been determined by the production of its own mines, which was low and steady. But the discovery of America and the rich and unexploited mines there had increased supply and decreased the value of metal in Europe by virtue of greater supply.
Precious metals also fluctuate little in value because they are not consumed: the corn grown one year will be consumed by the next, but all the gold ever mined in history is still available. "There is probably no other commodity subject to fewer variations." This, among the other properties of metal (durability, uniformity, divisibility) make it ideal as a basis of currency, and it has been recognized as such in all civilized and relatively advanced nations.
However, the value of metal, hence the value of money, does fluctuate, and as such it largely disqualifies money as a neutral arbiter of value across places and times, though its use in commerce might lead it to seem so.