>>37135
Well, value - or maybe worth - ofc a relative thing, frequently subjective, and couple of types of values exist. Something can have value for you but not for me. And price is also a different thing, while it's related. He >>37136 also gives good examples of stuff makes value relative.
A slab of rock in your example can have value if there's a demand for it.
Marx describes objective value, he says there's a - let's call it - "common denominator", which can help to determine the value of things objectively, and it's work.
For example diamond has more value than iron, since X amount of diamond to produce is more work than produce the same amount of iron (let's stay at uncut diamond, and iron ore, with the same amount of mining you get less diamond than iron, so the work divided per unit of diamond is more than in the case of per unit of iron).
And you can use it this to everything, a hammer consists of the head and the handle. Both needs to be made. Tree have to be felled, cut, handle shaped, it's all work. The ore has to be mined, made it into iron, then steel, then the bar have to be shaped into a hammer head, it's all work. But the tools which helps to making it, those also contain the work there were made. And the craftsmen knowledge, the learning process is also work.
So everything can be translated into work.
For example someone wants that slab of rock, just plain, no chiseling or sculpting. But to give it to him I have to move that thing and that's work.

The problem of intellectual products is, that how do I measure the work needed? Let's stay at philosophy for an example. One can come up with an ingenious idea effortlessly in the strike of a moment, other have to think hard for a lifetime for something else. Both really is a good thought, will the former worth less because there's less work behind it?