The diamond industry has a terrible problem. They promoted diamonds as "perfection". The most valuable diamond is supposed to be a crystal with no flaws. Then they ran into the semiconductor materials industry. Wafer fabs want every atom where it is supposed to be. So semiconductor materials manufacturers are very good at making crystals with few or no flaws.

So, now the synthetic diamond market looks like this.[1]

DeBeers has been frantically trying to stop the growth of the synthetic gemstone industry. (They lost on industrial diamonds decades ago; that's all synthetic and cheap.) DeBeers has an R&D program to develop test instruments. It is just barely possible at present to distinguish synthetic CVD (controlled vapor deposition) diamonds from real ones.[2] From that paper, it looks like some process improvement by the synthetic diamond makers could fix that. Detection involves exposing the diamond to a UV flash lamp, which makes diamonds fluoresce blue briefly, and examining the spectrum that comes back. Natural diamonds have a peak at 425nm, and some CVD synthetics have a peak at 435nm, at least for the ones tested.

There's a PR program for "natural diamonds". But there's also a PR program for synthetic diamonds.[3] The synthetics industry is gaining.

As an investment, diamonds are awful. They are really hard to sell.

[1] https://www.alibaba.com/showroom/cvd-diamond.html

[2] https://onlinelibrary.wiley.com/doi/10.1002/pssa.201800292

[3] https://www.cnn.com/style/lab-grown-diamonds-popularity-2023...