The challenge was that, according to Epstein, while diamonds were a rare resources only a couple of centuries ago, the discovery of extremely rich mines in South Africa and other countries of Africa was threatening to drive the prices down. The establishment of DeBeers consisted therefore, in a parallel effort, of setting up a cartel with other producers in order to control international prices of diamonds. A cartel is a loosely, but effective, agreement between producers of a resource, product or service to agree on minimum levels of prices offered for these resources, products and services to the general public. A cartel is usually also structured in such a fashion that its members follow the marching orders given by the cartel leader. The whole concept of a cartel is exactly what was the reason for the passing into US law the Sherman Act.
Cartels are immoral because they overcharge the buying public by artificially propping up prices.
They victimize the general buying public. Naturally the issues of the quality of products and truth in advertising are all rolled together into the issue of the victimization of the buying public. Products of a given quality are much too expensive for what they could be if the prices were allowed to be set by the market forces. Any advertising claims are relying on the concept that the products are attractive at their offered prices, and attractive for different psychological reasons for the buyers. It the prices are not morally set, the whole advertising exercise is an effort in deceit.