我把英文版的整段话贴出来
The value of a commodity would therefore remain constant, if the labour-time required for its production also remained constant. But the latter changes with every variation in the productiveness of labour. This productiveness is determined by various circumstances, amongst others, by the average amount of skill of the workmen, the state of science, and the degree of its practical application, the social organisation of production, the extent and capabilities of the means of production, and by physical conditions. For example, the same amount of labour in favourable seasons is embodied in 8 bushels of corn, and in unfavourable, only in four. The same labour extracts from rich mines more metal than from poor mines. Diamonds are of very rare occurrence on the earth's surface, and hence their discovery costs, on an average, a great deal of labour-time. Consequently much labour is represented in a small compass. Jacob doubts whether gold has ever been paid for at its full value. This applies still more to diamonds. According to Eschwege, the total produce of the Brazilian diamond mines for the eighty years, ending in 1823, had not realised the price of one-and-a-half years ' average produce of the sugar and coffee plantations of the same country, although the diamonds cost much more labour, and therefore represented more value. With richer mines, the same quantity of labour would embody itself in more diamonds and their value would fall. If we could succeed at a small expenditure of labour, in converting carbon into diamonds, their value might fall below that of bricks. In general, the greater the productiveness of labour, the less is the labour-time required for the production of an article, the less is the amount of labour crystallised in that article, and the less is its value; and vise versâ, the less the productiveness of labour, the greater is the labour-time required for the production of an article, and the greater is its value. The value of a commodity, therefore, varies directly as the quantity, and inversely as the productiveness, of the labour incorporated in it.

因此，如果生产商品所需要的劳动时间不变，商品的价值量也就不变。但是，生产商品所需要的劳动时间随着劳动生产力的每一变动而变动。劳动生产力是由多种情况决定的，其中包括：工人的平均熟练程度，科学的发展水平和它在工艺上应用的程度，生产过程的社会结合，生产资料的规模和效能，以及自然条件。例如，同一劳动量在丰收年表现为8蒲式耳小麦，在歉收年只表现为4蒲式耳。同一劳动量用在富矿比用在贫矿能提供更多的金属等等。金刚石在地壳中是很稀少的，因而发现金刚石平均要花很多劳动时间。因此，很小一块金刚石就代表很多劳动。杰科布怀疑金是否按其全部价值支付过。至于金刚石，就更可以这样说了。厄什韦葛说过，到1823年，巴西金刚石矿80年的总产量的价格还赶不上巴西甘蔗种植园或咖啡种植园一年半平均产量的价格，虽然前者代表的劳动多得多，从而价值也多得多。如果发现富矿，同一劳动量就会表现为更多的金刚石，金刚石的价值就会降低。假如能用不多的劳动把煤转化为金刚石，金刚石的价值就会低于砖的价值。总之，劳动生产力越高，生产一种物品所需要的劳动时间就越少，凝结在该物品中的劳动量就越小，该物品的价值就越小。相反地，劳动生产力越低，生产一种物品的必要劳动时间就越多，该物品的价值就越大。可见，商品的价值量与实现在商品中的劳动的量成正比地变动，与这一劳动的生产力成反比地变动。