The first uses of money basically involved tying the value of silver to the value of wheat (As in, the temples that were the center of commerce said 1 silver bar = 60 rations of food, and enforced that), and basically the silver bars never left the temple vaults for thousands of years because it was traded mostly in the form of the numbers written on the ledgers of accountants, rather than actually being traded around.
My history professor says your wrong. According to his class, an early Temple or Palace economy had no concept of currency whatsoever: they were pure communists, with everything going into and out of a common pot. The closest thing they had to currency was an ancient form of ration stamps. Later when empires rose they would find precious metals were convenient for paying officials, as being always in demand they could be traded for what was actually desired. Coinage came about as a means of storing metals in units of guaranteed weight and purity, and eventually trickled down into the middle classes during the Central-High Middle Ages. But even then, the concept of 'money' as we know it, a numerical measure of wealth, was still alien to them.
Yeah... I had to write an essay on this stuff last week.
Oooh, history lesson-off.
The earliest cultures were communist, yes, but I'm talking about the
introduction of metal-backed money. By definition, that means that they
were at the point they were using metal-backed money. By definition, that means before then, they weren't using metal-backed money.
Besides, you, yourself, just talked about how they were trading around ration stamps, which is exactly the money I was talking about.
The price of silver was tied to the price of grain to give silver a value - this was enforced by temples being willing to exchange the grain they were donated for silver if someone brought silver in. (The value of money being what it can be exchanged for, and the faith that it will be taken as legal tender.) The temples didn't actually circulate silver coins, however, they simply measured wealth on clay tablet ledgers. A farmer who got their tools worked on by a blacksmith would have to agree to pay that blacksmith back a given amount of grain when his harvest came in, and that debt would be recorded on a tablet, or the exchange would take place using their stamp-coinage.
You're wrong about the Middle Ages, however.
The "Pound Sterling", for example, was never minted for centuries after it came into use, and many medieval cultures continued to use the Roman Denarii as their measure of worth long after the Denarii had stopped being minted and stopped being in general circulation.
Only small change used by merchants would be handled in coins - most large amounts of wealth were still handled as ledgers in accounting books. Most nobles themselves were in debt, and the large currency exchanges were often just giant IOUs from some noble that was passed around the whole society.
Back to the actual topic at hand, however...
Platinum and Aluminum are overvalued and overly common as metals in-game.
Platinum only has value
now because it's extremely rare and actually
used for things.
Before, when it couldn't be seen as distinct from silver or tin, except that it was lower-quality, and where it was so rare as to make collecting it pointless, there would be no place you could have a guaranteed exchange of that metal for any given price, because nobody else would have heard of the metal.
As such, it is useless as money.
Without any of the later, more modern uses, there would be no demand for it, either. As such, it would be useless as a trade good.
Without people recognizing it, it would be useless even as a rare thing for nobles to want just to show off how they can get something rare.
Platinum would therefore have no value.