Whenever I think about the diamond market I think about the episode of the Twilight Zone where gold becomes worthless in the future because it's manufacturable.
As evidence by the diamond market, even if we could manufacture gold (somehow) even higher in quality than the purest gold available naturally, people would still want the, objectively worse, natural stuff, because for some reason the human rights violations that go into getting it makes it more valuable.
TL;DR: it's the child labor that makes it special