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Awesome, Stefano! Thanks for jumping into the discussion!
I think that maybe the answer is included in the question which asks about the difference in how expensive one substance is when compared to the other. Expense refers to price which is a function of value, right? But values are subjective, hence there isn’t just one price for something, but various prices depending on the parties who are trading. So why would someone value an ounce of diamonds more than an ounce of water?
I think that part of the equation has to do with the relative scarcity of each item (one being scarce and the other not for most people).
Might it also have to do with the amount of work that goes into each (does that relate back to scarcity?): water requires some treatment (unless one finds an artesian well or other clean source straight from nature, but how rare is that, anymore?) and the diamond requires a lot of work to dig from the ground and to make it either pretty for jewelry or useful for industry. Water treatment benefits from economies of scale, thus making the next ounce less expensive than the first treated, but diamonds don’t benefit from those economies. (But the amount of work that’s gone into something isn’t always going to make that something valuable to someone else. Two chefs, one really good, one not so much; two meals, both with a lot of work, one worth buying, one not. (I think I think too much 😊).)
However, as we’ve both mentioned, values are subjective and can change. I know someone who was stranded in the desert when his car died on a day that reached 122˚F (pre-cell phone); he was grateful that he hadn’t grabbed a 5-gallon jug of water, but three 1-gallon jugs, because a 5-gallon jug would have been too heavy to carry as he and his companion walked to where they could get help. In that case, more water wouldn’t have been better (but diamonds would have been useless). So values change.
Am I close?