Christie's, which had refused to reveal the name of the seller, said only four vivid pink diamonds or over 10 carats have ever been offered at auction.
Christie's did not identify the jewel's latest owners but it once belonged to the Oppenheimer family, who ran the De Beers diamond mining company for three generations until selling their stake in 2011.
And a diamond like the famed Pink Legacy - which is extraordinarily vivid for such a large diamond, weighing in at a whopping 18.96 carats - is so unusual that it has been called "as good as it gets" for a colored diamond, according to Rahul Kadakia, International Head of Jewellery at Christie's auction house.
Although Christie's did not reveal the auction victor, the diamond once belonged to the Oppenheimer family, who ran the De Beers diamond mining company until 2011 before selling it to Anglo American, a multinational mining company based in Johannesburg, South Africa, and London, UK.
The stone was immediately rechristened the "Winston Pink Legacy" by its buyers.
Rahul Kadakia, Christie's head of worldwide jewelry, said that auction house has only sold four pink diamonds weighing more than 10 carats in its 251 years of history.
Christie's auction house jewellery specialist Rachel Evans-Omeyer poses with an 18.96 carat Fancy Vivid Pink Diamond on show in London on September 25, 2018 before its November auction with an estimate at U.S. $30 to $50 million.
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Christie's sale kicks off two days of jewellery auctions in Geneva.
The stone was discovered in a South African mine around a century ago and was probably cut in the 1920 and has not been altered since, Christie's said.
Fancy Vivid Pink diamonds over 10 carats are extremely rare.
Christie's had expected to fetch between 30 and 50 million dollars for the almost 19-carat, rectangular-cut stone - the largest fancy vivid pink diamond it has ever auctioned.
"I therefore believe the Pink Legacy will beat the current world auction record", he added.
"It is probably the most lovely (specimen) ever presented at public auction", Lunel said.