It isn't always the cartel's goal to drive prices up. In the case of de Beers, they wanted price stability. They went out of their way to avoid increasing price

It isn't always the cartel's goal to drive prices up. In the case of de Beers, they wanted price stability. They went out of their way to avoid increasing prices more than the desired amount at many points in the history of consumer diamonds. There is no reason to believe that the price of diamonds at the time De Beers lost control was not at approximately what the market could bear, especially now that they have liquidated their supply-controlling stockpile. Growing demand from developing countries and speculative pressures that would not have existed before could certainly account for increasing diamond prices.