Interesting doc on Netflix called Nothing Lasts Forever goes into great detail on the diamond cartel and how 'synthetic' diamonds are getting mixed in with mined ones.
Yup, it's been extremely effective. I got engaged at the end of last year and certain types of people seemed genuinely horrified that I didn't get a diamond ring.
Anecdotally though those tended to be older folks so I think maybe it's losing effectiveness and younger people are seeing how absurd and arbitrary the whole thing is.