That’s an interesting analogy, I’m not really sure how to answer that. I suppose I would respond by saying that water isn’t exactly analogous because it’s not a luxury good. A better example might be a “fake” Rolex or a “fake” Hermes handbag. Both such items can be convincingly faked (at least up to very close review, even if not by a qualified inspection team with knowledge of trade secrets). But people still prefer the “real” luxury items.

Preferring the “real” over the “fake” is only irrational insofar as preferring either is irrational. You either buy these items because you want the status signalling effect, or yoy buy them because you actually have an innate appreciation for them.

I think the gp's analogy of natural water is better than the luxury good you suggested because the underlying distinction is a choice between something naturally occuring vs man-made.

I disagree. Gold is clearly a luxury item despite being natural. More importantly, diamonds are luxury goods, for better or worse, which means they wouldn’t adhere to the price dynamics of water.

Not sure what the source of disagreement is, particulary when my comment was merely to agree with the gp's water analogy when s/he asked:

"What makes a diamond "natural"?"

Diamonds are perceived as luxury goods because of the elaborate fiction of artificial scarcity woven around it by De Beers, which you have acknowledged upthread.

Mentioning gold wasn't necessary because alchemy never delivered on a man-made substitute for gold. And it distracts from the core issue, we are here debating the consequences of the Chinese lab's feat for diamond after all.

EDIT: To summarize, I'm not clear on why you think applying a "luxury goods" lens will lead to better insights as to why people prefer "natural/real" over "man-made/fake" substitutes, when the underlying issue is one of perception, which can be shaped by anyone with an agenda and a marketing budget.