Cecil Rhodes, an English born South African business man, bough the mine owned by the "de beers" brothers in 1880 and it wasn't long before he owned virtually all of the diamond mines in South Africa. In 1888, De Beers Consolidated Mines, Ltd. was formed. They began crafting a monopoly on production and distribution of diamonds. In order to control supply and demand ("price making") he created a scarcity of diamonds. And by the time Rhodes died, De Beers owned around 90% of the world's diamond production and distribution.
Ernest Oppenheimer, owner of a rival diamond producer, essentially bought his way onto the board of De Beers and eventually became the Chairman of the Board. He established exclusive contacts with diamond suppliers and buyers, making it impossible to deal with diamonds outside of De Beers.
In the 1930's, a decrease in diamond sells caused Henry Oppenheimer, Ernest's son, to begin a campaign to increase the number of diamond engagement and wedding rings. In 1947, he coined the slogan " A Diamond is Forever" which eventually became the companies slogan.