This is very similar to fashion in the amount of sense it makes as a consumer. The reason this is more interesting is because of how striking it is as what essentially amounts to a conman's (DeBeers et al) success story, on a gargantuan scale. On top of that, people who buy into fashion are pretty much aware of what's going on, but I'd be willing to bet that a LARGE amount of people (myself included, until ~my late teens) are unaware that so many of the "hallowed traditions" surrounding diamond rings are marketing nonsense dreamed up a few decades ago.