The problem is that a diamond is inherently a basically worthless rock, so its retail value is already a figment of the public's imagination. Blue Nile is the closest that exists today, but they are not that disruptive since the retail channels already have lots of competition -- it's the supply (De Beers) that's monopolized.
The real disruption will come in the form of widely available manufactured (not mined) diamonds.
I used to think of diamonds as basically worthless rocks, until I went shopping for an engagement ring stone and looked at them under the jeweler's microscope, and thought "I want to be dressed from head to toe in these things". With mass-produced diamonds, that may become a reality.
Yes, they're nice to look at, but unlike, say, emeralds, they're not very rare -- they're simply controlled by a monopoly supplier. Manufactured diamonds will bring back a competitive supply which would exist if there was a healthy assortment of companies in the diamond mining industry today.
