Mining company Anglo American has gained a controlling interest in diamond miner De Beers, paying $5.1 billion for the 40 percent of De Beers shares held by the Oppenheimer family.

The deal potentially raises Anglo American's stake to 85 percent. The government of Botswana, however, has pre-emption rights to buy one-fourth of the Oppenheimer shares at the time the transaction closes, potentially increasing its stake to up to 25 percent.

"This has been a momentous and difficult decision as my family has been in the diamond industry for more than 100 years and part of De Beers for over 80 years," said Nicky Oppenheimer, representing the Oppenheimer family interests which are held by CHL Group.

Anglo American shares were up 3.4 percent in midmorning trading in London.