Serious post: if you plan on buying a diamond "as an investment", consider not doing that.

Humans can produce lab diamonds at 10% of the price and higher purity than natural , and I expect the process to improve on both axes. And making a synthetic diamond dirtier is way easier than purifying a natural one. There's also some developments in making quite big synthetic diamonds.

So yeah, if you're buying a diamond *as an investment*, you're speculating on market irrationality.