Diamonds aren't actually worth 2m. What you do is you take a 1.8m insurance policy against it and you trade the policy with other speculators on how much you can sell the diamond for.

Art works similarly. It's a finite commodity and so you take our insurance policies and auction it off and then you can speculate on the value of the work since you can then lease it out for shows, or to people who have the money to rent art.