Not an atom of matter enters into the objectivity of commodities as values;
in this it is the direct opposite to the costly sensuous objectivity of commodities
as physical objects. 
                                        –Marx, Capital I

David Harvey doesn’t like Wittgenstein, but Harvey could be paraphrasing Wittgenstein when Harvey interprets Marx as saying “value is immaterial but objective.“ That is, Wittgenstein would argue that, say, a painting’s value isn’t in the painting itself, but rather in the response to the painting, in the language game(s) that determines a painting’s aesthetic qualities and other merits. The relation is immaterial in that one cannot say exactly what it is (per Marx materially, atomically; per early Wittgenstein the nature of the form itself–the nature of, say, a painting’s likeness to what it represents). Money gives this immaterial relation, like any commodity, its objective nature.

If there is a difference between the two, it must be that Wittgenstein is concerned with the commodity’s effect (i.e., pragmatic response or “nonsense” that can only be shown), while Marx is interested in the social process bearing the commodity and the exchange process when “abstract and concrete labor come together.” Yet these processes make value immaterial but objective.

It is interesting to note that some orthodox Marxists believe Harvey here to be suggesting that value isn’t sourced in labor (LTV), which is material. Harvey, however, knows that Marx
does not mean to say that immaterial social relations don’t contain very real, material
bodies. For Marx clearly states, “Now we know the substance of value. It is labour.
We know the measure of its magnitude.
It is labour-time” (Capital I). Value
as a social relation contains both fixed
capital that includes machines and factories and the variable capital of human workers, and these are material in nature. But for Marx value is historical and contextual, too,
a social relation that one cannot get
to or contain in any absolute sense. It is in fact money that gives this immaterial social relation its objective form. Money is the objective representation of value, and value, again, an
immaterial but objective social relation. Money not only measures what is
otherwise unmeasurable and hence gives value its objectivity, but money appropriates
social wealth as individual property (Harvey).

Money “congeals”–conceals–the immaterial social relation, that is, when “exchange processes produce a representation of value in the money commodity, the money form, the universal equivalent” (Harvey). 

There is, however, a difference between material wealth and value:

“And the first thing [Marx] notices [ … ] is that ‘an increase in the amount of material wealth may correspond to a simultaneous fall in the magnitude of its value.’ Value is dependent upon human productivity. Highly productive people can produce a large amount of material wealth
very quickly. And they can work less hours, so actually the amount of value that they make can be very low but the amount of material wealth they generate can be enormous. So again, he’s going to emphasize that distinction between material wealth and value. And he goes on to point out that while changes in productivity affect material wealth, they don’t necessarily have any effect at all on value creation. We will see instances where this is the case, but,
nevertheless, the change in productivity
is itself not directly connected to transformations in value” (Harvey).