In describing Marx's conception of value and wealth, it is very important to note that he has the two-fold concept: wealth is use-value and exchange-value. Nature gives us use-values, but no exchange-values. Human labor is the "onliest" source of EXCHANGE-value, for Marx...although , we do have bubbles.
CB
Value
A commodity is, in the first place, a thing that satisfies a human want; in the second place, it is a thing that can be exchanged for another thing. The utility of a thing makes is a use-value. Exchange-value (or, simply, value), is first of all the ratio, the proportion, in which a certain number of use-values of one kind can be exchanged for a certain number of use-values of another kind. Daily experience shows us that million upon millions of such exchanges are constantly equating with one another every kind of use-value, even the most diverse and incomparable. Now, what is there in common between these various things. things constantly equated with one another in a definite system of social relations? Their common feature is that they are products of labor. In exchanging products, people equate the most diverse kinds of labor. The production of commodities is a system of social relations in which individual producers create diverse products (the social division of labor), and in which all these products are equated with one another in the process of exchange. Consequently, what is common to all commodities is not the concrete labor of a definite branch of production, not labor of one particular kind, but abstract human labor-human labor in general. All the labor power of a given society, as represented in the sum total of the values of all commodities, is one and the same human labor power. Thousands upon thousands of millions of acts of exchange prove this. Consequently, each particular commodity represents only a certain share of the socially necessary labor time. The magnitude of value is determined by the amount of socially necessary labor, or by the labor time that is socially necessary for the production of a given commodity, of a given use-value. "Whenever, by an exchange, we equate as values our different products, by that very act, we also equate, as human labor, the different kind of labor expended upon them. We are not aware of this, nevertheless we do it." [Capital] <http://www.marxists.org/archive/marx/works/1867-c1/index.htm> . As one of the earlier economists said, value is a relation between two persons; only he should have added: a relation concealed beneath a material wrapping. We can understand what value is only when we consider it from the standpoint of the system of social relations of production in a particular historical type of society, moreover, or relations that manifest themselves in the mass phenomenon of exchange, a phenomenon which repeats itself thousands upon thousands of time. "As values, all commodities are only definite masses of congealed labor time." [A Contribution to the Critique of Political Economy] <http://www.marxists.org/archive/marx/works/1859/critique-pol-economy/index. htm> . After making a detailed analysis of the twofold character of the labor incorporated in commodities, Marx goes on to analyze the form of value and money.
http://www.marxists.org/archive/lenin/works/1914/granat/ch03.htm#s1.1