It is only the commodities that change hands as they are exchanged according to the value that is given to them by a fetishizing desire. Because commodities can be exchanged for one another, Marx believed that all commodities have something in common (and one of the most interesting aspects of commonality is the fact that nearly all of the producers of commodities remain, for the most part, invisible). This commonality cannot be ascribed to their use value because they differ in quality. Marx drew the conclusion then that capital (in general) and fetishism (specifically) "must be equal to a third, which in itself is neither the one nor the other" (Marx). This third thing is different from the use value of the goods concerned and thus also from that which provides these goods with their visibility and materiality and makes them useful with respect to the satisfaction of consumer needs. It remains invisible and becomes a sort of abstraction and escapes the consumer's awareness.

The value of commodities is quite illogical and incredibly random, according to Marxist theory. A gemstone is more valuable than a carton of eggs, for example; however, the carton of eggs is by far more valuable in terms of practical living because food is something we require to live; a gemstone is not needed for anything but status, greed, etc. Capitalism fetishizes commodities and imbues them with values that are way out of proportion to their function or usefulness to individuals or communities.

The illusion that value comes from the commodity itself and not from the social relations behind it is a fetish. Marx believed that a capitalist society is full of these types of illusions....

Money, for example, is considered the most valuable in qualities, but it is only because it is used to express the value of all other commodities. Profit may seem to come out of exchange, but profits actually originate in production through the unequal relations between capital and labor in the work environment.
Since the producers do not come into social contact with each other until they exchange their products, the specific social character of each producer's labour does not show itself except in the act of exchange. In other words, the labour of the individual asserts itself as a part of the labour of society, only by means of the relations which the act of exchange establishes directly between the products, and indirectly through them, between the producers (Marx 321).

Fetishistic ideas are so common nowadays especially in the notion that value comes from the subjective experience between a consumer and a commodity, and that capital creates value all by itself.

Marx's theory of fetishism is about the way social relations between people take on a material form that then act back upon and shape these social relations. Labor takes the form of value personified in commodities. Money becomes the common manifestation of the value. The pursuit of wealth in money terms is something that controls our culture. Money, commodities and capital, as symbolic of social value, become self-governing powers out of the control of society. Attempts to exert some control over these forces through domination or the state always become trapped in the social opposition of value.

Reference

Marx, Karl. (1887). Capital, Volume I. pp. 294-365.