Worth is established merely by being trade-able.

Credit is not concrete, it is merely agreed on. In any trade you essentially agree that the value is fair.


The simple example is bitcoins. They exist in a closed system and they require a specific amount of effort to acquire. Essentially we have a very limited version of other currencies - in which there is a fluctuating amount of currency, and jobs are performed to acquire currency.
Bit coins are inflated when more people join the mining, thus making it more competitive and harder to acquire coins. Additionally the more people that trade in bit coins inflate the value further merely because there is a higher proportion of users per coin.
Mid '11 a lot of people heard about bitcoins and jumped in, buying hardware to support mining, and because of the massive influx of users the value inflated to more than tripple it's previous value. This level was artificial, it was merely perceived that they were worth three times their value, and after the value crashed it settled to around 50% more than the original value.

bitcoins are probably a good platform for studying currency dynamics because the system is closed and limited. Effects of new trades, security breaches, mining influxes, technology improvements can be seen plainly.