I think the next few years should be OK for diamond miners. I own a relatively small basket; a mid-tier producers plus two related explorers, but nevertheless the positions are highly speculative.
The thesis is relatively simple, and therefore hiccups should be easy to spot. About 20% of global supply went offline last year, in part because of Rio's closure of its Argyle mine which accounted for almost half of the reduction, and the remainder due to covid. Any additional supply that can be bought on line in the near future will be incremental and from existing mines only, as there haven't been any significant finds of late. The majors have also indicated they will do more to control supply as in the past.
It is worth mentioning through that De Beers has initiated a $6B expansion on its mine, and Lucara (which I own) is mid way through its $500M expansion. Rio Tinto recently settled with its JV partner regarding explorations costs that will run to around $150M, which all points to some initial positioning. Alrosa - the biggest producer by volume - has also announced plans to expand. Despite this, Bain still seems to think (and most miners have agreed) that the additional supply will not be able to keep up with demand and depletion.
I own LUC because its got the best economics (financial and mine) amongst its mid-tier peers, and earns the second highest $/carat behind GEM. GEM however hasn't been able to do anything of note with its profits, whilst LUC has been able to atleast return what was initially invested in the business via dividends.
In a good market it appears capable of generating $100M+ in FCF, against a MC of $270M. Obviously this will not end up in the hands of us investors in the near future given the expansion, but it will result in the mine life being extended from 2026 to 2040, so whether or not the market rewards this in some share price appreciation remains to be seen.