Hot take: it's not going to stop at diamonds. Marriage is expensive. Weddings are expensive. Honestly, if you compare it to the other really expensive things that happen around the same stage of life... buying a home, having children... Unless I'm from a highly religious family(also in decline), I can tell you easily which one I'd call pass on. https://ifstudies.org/blog/1-in-3-a-record-share-of-young-ad...
I've read that a century ago, about a third of engagement stones were precious stones other than diamonds. So, a triumph of marketing.