We can already create synthetic diamonds [0] and there is no market for them, at least not as gemstones. People seem to want 'the real thing'.
Gold, on the other hand, is routinely melted down and recast.
Correction; women seem to want 'the real thing', but diamonds aren't themselves valuable, they are a status symbol. Girls don't want to show their friends a "fake" diamond and unfortunately due to marketing, they consider man-made fake even though it's real.