People buy diamonds for a lot of the same 'irrational' reasons they believe in religion. Brainwashing. Girls have it pounded into their head over and over all their lives that this diamond is a MUST so they never think with their own brains about it. 99% of people would never spend $10K on a single piece of jewlery, if it was logic and reason, as opposed to pure brainwashing, that was governing their decisions. So actually it's a very sad aspect that I hate about humanity. Susceptibility to being brainwashed and the need to fit in, and the fear of not fitting in. All the above are mental disorders. And by the way just because "everyone is doing it" doesn't mean it's not a mental disorder. It is.