De Beers enjoyed a monopoly on the market for a long time, so the prices were pretty much standardized across the entire marketplace. Yes, you could obviously find someone willing to sell you diamonds for more than what De Beers charged, but nobody would believe that you willingly paid more than "market rate". "Market rate" in this context refers to De Beers' monopolistic pricing, not actual market rate, which is actually peanuts because diamonds are in fact very common.