I saw a good response to this on Tumblr once.

Tl;Dr version, "Diamonds aren't even really rare. We have tons of them, and just let a few out at a time to make them expensive. Also, in a study, a majority of people believed regular clear quartz was real diamond. In other words, diamonds are bland, actually valueless clear gems that look like much more dull gems and you're much better off buying a pretty gemstone like Emeralds or Sapphires, ect."

It's hilariously, yet despairingly true. Also why would anybody buy raw, crude diamond? There's no application for it except "sup bitches, look at my fucking diamond".