Karl Marx based his claim that all workers are exploited within capitalism, upon the assumption that the value of a product is basically determined by the amount of labour put into it. More specifically, the value of a product equals the cost of raw materials, usage of means of production (both factors are more or less constant) and then extra value added to it because of labour (e.g. coffee is worth more than the mere sum of water and coffee beans; the extra value is created by grinding the beans, ...) The point then is that a worker's wages are typically less than the extra value of the product added by labour. Hence the worker is performing some of the labour for free, hence exploitation. Now my question: I can see the reasoning behind this, but I question the assumption that the value of the product is determined by the amount of labour put into it. For example, suppose one worker is digging for gold, while another one is digging for stone. Let's say the labour they do is roughly comparable, yet surely the value of the product will be much higher in the first case? Does this invalidate Marx' theory, meaning that workers are actually not exploited? Probably not, but where do I go wrong?