Because of it's hardness diamonds are used in drills and many other things. They also conduct heat efficiently. If you could make glasses out of them they would be thinner than glass ones and much more scratch resistant. There is even the possibility of very high temperature transistors made out of diamond for when you need control equipment in hot places.
I should’ve specified just used for jewelry. They have no practical use. Why do they cost so much, and kept artificially scarce?
Questions I’m not sure I want the answers to.