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.
Also part of the marketing is a lot of "helpful" educational material teaching the public about the "4 C's" -- color, clarity, cut and carat weight -- that determine the quality of a diamond, to make sure people know they should be insulted by $500 "I-2" diamonds and start debating whether they'd settle for a "VS-2" in place of a "VS-1" when they previously couldn't tell the difference by looking.
Because this is totally how you will be looking at your ring every day.
Sure, at the lower levels, you can actually see some of the differences without magnification, like some low-grade diamonds are obviously pee yellow or look like they've been hit with a baseball, but squinting at a diamond for minor cloudiness and number of facets is something the average person would never have thought of doing, or being upset about, until the salesman helpfully guides you there.