The floating of the value of exchange goods depend on demand and supply. For instance, the exhaustion of a gold mine makes the value increase since the following decreasing of the supply. Money can be anything the parts consider exchangeable but not all the goods are practical at the same way.
Money must be storable for long periods, easily transportable and not too easily available. Once again, demand and supply have a key role to determinate the value. If a government prints more banknotes, it increases the supply without increasing value. Money has less value than the new banknotes.
Metals such as gold and silver have been often used as exchange goods. To better the characteristics and the workability, these metals often are used in alloys with less precious metals, making their value very variable.
