Real Money
item of exchange,
The Indians of the american interior used Shells, the south sea islanders used carved Stones, People in a poker game, use Red, White & Blue pieces of plastic. As long as everyone agrees ...........
Measure of Value
If a Pig costs 1 stone and a cow costs 2 stones, whe can easily see that the cow is twice as valueable as Pig.
Store of Value.
If I sale my cow for two stones, in the fall and buy a pig for one of them, I expect that in the spring, with The 1 extra stone will still be able to buy a pig.
If the Store of value is stable enuff*, I would be able to do this several years later, instead of just the next spring.
This is where the Rareity, of the item used, comes in.
Wampum was Ocean Shells hard to come by in the interior. Carved stones took a lot of time & work to produce. Metals like Gold Silver and Copper in Europe and Asia, Take a lot of work to produce, and are rare enuff that you can't just run out and get some more, whenever you want*.
There were early Bronze and Iron coins, but as bronze and iron lost their rareity, so did the Coins lose their value.
With modern industrial processes, there is a scariscy of Rare items, suitable for Items of Exchange. This is why most future SciFi stories never mention what the "Credit" most use, is based on.
*Failure to satify parts 2 & 3 is Why paper currency is not Real Money