Diamonds are used for a great many high-tech purposes like lasers, although in 200 years those uses may be obsolete.
The big advantage of diamonds is that you can easily carry several millions of dollars worth in your pockets.
Consider that aluminum was once so rare that the Washington Monument has a 4 inch aluminum pyramid at it's peak, since at that time it was the most valuable metal in the world. Diamonds will be valuable only so long as DeBeers can maintain the monopoly. I suspect that diamonds will be worth the same as aluminum in a few years, and WAY less than 200 years.