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.
You're probably right, but who knows what gold or platinum will be worth in 200 years.