Commodity money cans or foil pouches of mackerel fish fillets, which have a fairly standard cost and are easy to store. These may be exchanged for many services in prisons where personal possession of currency is prohibited. In situations where the commodity is metal, typically gold or silver, a government mint will often coin money by placing a mark on the metal that serves as a guarantee of the weight and purity of the metal. In doing so, the government will often impose a fee which is known as seigniorage. The role of a mint and of coin differs between commodity money
Commodity goods are also treated as commodities, e.g. human labour-power, works of art and natural resources ("earth itself is an instrument of labour"), even though they may not be produced specifically for the market, or be non-reproducible goods. Marx's analysis of the commodity is intended to help solve the problem of what establishes the economic value of goods, using the labor theory of value. This problem was extensively debated by Adam Smith, David Ricardo and Karl Rodbertus-Jagetzow among others. All three of the above-mentioned economists rejected the theory that labour composed 100% of the exchange value of any commodity. In varying