If you hang around with cynical people who are always making a big deal about how Mother's Day or Valentine's Day were invented by Hallmark to make money, you'll probably have heard the whole spiel from them about how the De Beers diamond cartel has manufactured the tradition of diamond engagement rings and relentlessly pushes it on people.
Spontaneous profession of love or slavish acquiescence to The Man's propaganda?
If you haven't, you can get caught up by reading that link. While the "DE BEERS IS BEHIND EVERYTHING" rhetoric can get a bit hyperbolic, there's some truth to it. There are a whole bunch of "have tos" society seems to assume about engagement rings -- it has to be diamond, you have to spend two months worth the salary on it, you have to use it to propose. These rules don't really make sense and seem to trace back to marketing rather than genuine traditions. The phrase "diamonds are forever" isn't even a colloquial idiom but a marketing slogan invented in 1948 as "A Diamond is Forever."
I kind of wish all James Bond titles came from advertising slogans.