Rarity of diamonds realy is exaggerated a bit, but company induced price gouging is not the only reason they're expensive. Yes, there's a world monopoly on diamonds by De Beers, and they're as cutthroat a company as it can get, but the main reason diamonds are so expensive, is because good diamonds are rare as fuck. Although diamonds are not insanely rare as one might think (most rare earth metals are insanely rarer and difficult to get), jewel-grade diamond is, in fact, rare as fuck. For a diamond to be crystal-clear and perfect, it has to form deep underground, under massive amounts of pressure and heat, but at the same time, somehow, keep away impurities, which is notoriously difficult, since it's forming in the middle of the goddamn earth inside a pocket of lava that's being compressed and churned around. Yeah. To further compound the issue, these occurrences tend to give formation to small jewel grade diamonds, not the big rocks you find used in the insane-expensive jewelry. So, not only most of the diamonds are small, they're also impure, which makes finding the really good really pretty diamonds something hard as fuck. Which drives the price up. Not only that, even when you have a big enough diamond you can make a pretty jewel out of, you still have to chisel most of it away. Meaning even bigger diamonds are needed. It's not just impurities, either. Cracks in the crystalline lattice structure can and will change the outer look of the diamond, and sometimes alter the way the light refracts on the diamond, changing it's color as well, giving it an array of hues (some of which are even rarer and sought after than crystal clear diamonds). Diamonds for industrial use, however, are very much easier to come around, especially since most of the time you only need a shitton of really small diamonds to make a coating for something.