To add to my comment a little, and this was touched up in the article. The vast majority of rough diamonds are sent to India for cutting/polishing, before being sold to the retail channels (Tiffany, Zales, Wal-Mart, etc). The profit margin for the miners is rather small, the profit margin for the cutters/polishers is razor thin, and the profit margin for the retailers is HUGE.
Just speculating here, maybe if you fly to India and visit their cutting shops in Surat.
The fairest prices as I understand it are on BlueNile, which charges about 10-15% above cut price, which is much better than the 100+% you'll find in retail.