If you think about it, it wouldn't make much sense to take something worthless and ubiquitous and try to make it artificially rare and valuable, that would be extremely difficult. Rather, it would make more sense to take something already rare and valuable and make it more so by cornering the market. Indeed, the only reason the market would be able to be cornered is because of its rarity.

And that's the situation with diamonds. The De Beers diamond cartel has resulted in inflated prices of gem stone diamonds but they have always been fairly rare and valuable. The last time I did some digging I came up with something like a 3x price multiplier that De Beers has managed to achieve. The more remarkable aspect of De Beers is protecting diamond prices from the influence of production changes caused by technological improvements. Diamond prices have not only remained fairly steady over time but have increased relative to inflation (due to increased demand and increased per capita GDP). In a free market diamond prices would have fluctuated wildly and would likely be lower than they were over a hundred years ago due to improvements in production.