More On Commodities

Having magically discarded unwanted use-value and done the cookery to homogenise, congeal and crystallise labour into a commodity, Marx sets up a nice straw man:

A use-value, or useful article, therefore, has value only because abstract human labour is objectified or materialised in it. How then is the magnitude of this value to be measured? By means of the 'value-forming substance', the labour, contained in the article. This quantity is measured by its duration [in] hours, days etc.

Cue an internet meme about, who wants to live under a system where the value of something is greater the slower and dumber the producer?

The straw man is duly knocked down, making Karl appear entirely reasonable:

It might seem that if the value of a commodity is determined by the quantity of labour expended to produce it, it would be the more valuable the more unskilful and lazy the worker who produced it

sounds like Britain in the 70s... but it

only needs, in order to produce a commodity, the labour time which is necessary on an average, or in other words is socially necessary. Socially necessary labour-time is the labour-time required to produce any use-value under the conditions of production normal for a given society [...] The individual commodity counts here only as an average sample of its kind.

Marx goes on to ponder, in the first real-world example cited in the book, that:

The same quantity of labour provides more metal in rich mines than in poor. Diamonds are of very rare occurence on the earth's surface, and hence their discovery costs, on an average, a great deal of labour time [... William] Jacob questions whether gold has ever been paid for at its full value [...] The total produce of the Brazilian diamond mines for the eighty years ending in 1823 still did not amount to the price of 1½ years' average produce of the sugar and coffee plantations of the same country, although the diamonds represented much more labour, therefore more value.

Karl Marx "Capital Volume I" (1990) pp129-30

Could the explanation be that the theory is a hopeless hack that has fallen at the first fence?

Or is this a Conspiracy Theory in the making? I will read on with my expectations raised as high as my hackles.