Anglo American takes control of world’s diamond resources

A key Oppenheimer strategy was to monopolise the mining industry by acquiring established mines. Such domination was performed in the diamond industry by taking advantage of the Great Depression to acquire majority ownership of Cecil Rhodes’ diamond empire De Beers, and Ernest Oppenheimer became its chairman.

https://londonminingnetwork.org/2016/02/londons-mining-history-from-colonialism-to-apartheid-why-rhodes-must-fall/

Innes, Duncan. Anglo: Anglo American and the rise of modern South Africa. Raven Press, 1984.