Purposefully weak, to my understanding, to discourage a secondary market.

I remember an article years ago that went into detail about De Beers' manipulation of the market, and how change from "larger is better" to "clearer is better" happened just as a large mine outside of De Beers control opened that had quite a few large diamonds coming from it on a regular basis. That same article made some assertions about the secondary market being somewhat manipulated, IIRC, but this was close to a decade ago, so sourcing it might be hard, and I'm also unsure how accurate it was at this point. I think it was about how modern incarnation of idea of giving rings for marriage was also largely manipulated by De Beers.