Commodity money is any money that is both used as a general purpose medium of exchange and as a tradable commodity in its own right.

Commodity based currencies are often viewed as more stable, but this is not always the case. The value of a commodity based currency as a medium of exchange depends on its supply relative to other goods and services available in the economy.

Historically, gold, silver and other metals commonly used in commodity based monetary systems have been subject to regular and sometimes extraordinary fluctuations in purchasing power. This not only damages its stability as a medium of exchange; it also reduces its effectiveness as a store of value. In the 1500 and 1600's huge quantities of gold and even larger amounts of silver were discovered in the New World and brought back to Europe for conversion into coin, the purchasing power of those coins fell by 60% to 80%, i.e. prices of commodities rose, because the supply of goods for sale did not keep pace with the increased supply of money.[3] In addition, the relative value of silver to gold shifted dramatically downward.[4] More recently, from 1980 to 2001, gold was a particularly poor store of value, as gold prices dropped from a high of $850/oz. to a low of $255/oz. The advantage of gold and silver, however, lies in the fact that, unlike fiat paper currency, the supply cannot be increased arbitrarily by a central bank.

It is also possible for the trading value of a commodity money to be greater than its value as a medium of exchange. When this happens people will often start melting down coins and reselling the metal used to make them. This has happened periodically in the United States, eventually causing it to move away from pure silver nickels and pure copper pennies.[citation needed] Shipping coins from one jurisdiction to another so that they could be reminted was sometimes a lucrative trade before the advent of trusted paper money.[citation needed]

Commodity money's ability to function as a store of value is also limited by its very nature. Copper and tin risk rust and corrosion. Gold and silver are soft metals that can lose weight through scratches and abrasions.

Stability aside, commodity based currencies are limiting in a rapidly growing or very active economy. The supply of money in an economy must be equal or greater than the volume of trade. If commodities are used as money, then the money supply must equal the total amount of goods and services sold. In a large economy, the volume of trade can easily outstrip the supply of any one commodity.

This problem is compounded by the fact that money also serves as a store of value. This encourages hoarding and takes the commodity money out circulation, reducing the supply. The supply of circulating commodity currency is further reduced by the fact that commodity moneys also have competing non-monetary uses. For example, gold and silver is used in jewelery and nickel and copper have important industrial uses.

Commodity based currencies also limit the geographic extent of the trading market. To make large purchases either a large volume or a high weight or both of the commodity must be transported to the seller. The cost of transportation of the currency raises the transaction cost and makes long distance sales less attractive.