Anglo American is to take control of diamond company De Beers after buying the Oppenheimer family's 40% stake for a $US5.1 billion.
The deal raises Anglo's stake in the world's largest diamond distributor to 85%, with the remainder owned by the Botswana government.
The Oppenheimers have been in the diamond industry for about 100 years.
Nicky Oppenheimer, representing the family, said it was a "momentous and difficult decision" to sell the stake.
Anglo chief executive Cynthia Carroll said it was a unique opportunity for Anglo American to consolidate control of the world's leading diamond company.
The BBC reports it has long been speculated that Anglo might increase its stake in De Beers, which vies with Russia's Alrosa for dominance in the diamond mining industry.
De Beers was founded in 1888 by Cecil Rhodes. Ernest Oppenheimer, who earlier founded Anglo American, took control in 1927.