This "Pink Legacy" diamond has set a new world record after going up for auction at Christie's.
Christie's sale kicks off two days of jewellery auctions in Geneva.
Pink Legacy is categorized as a "Fancy Vivid" diamond, the highest grade of color intensity. That amounts to $2.176 million per carat, which was previously the world auction record price per carat for any pink diamond.
The company promptly renamed it "The Winston Pink Legacy" and added it to its collection of rare jewels, the auction house said.
Although it was once owned by the Oppenheimer Family, who built De Beers into the world's biggest diamond trader, the identity of the seller was not disclosed.
The stone was discovered in a South African mine around a century ago and was probably cut in the 1920s and has not been altered since, Christie's said.
The auction house had expected to fetch $US30 million to $US50 million for the nearly-19-carat, rectangular-cut stone, the largest fancy vivid pink diamond that it has ever put under the hammer.
"The previous record was $2.1m (£1.6m), but for a much larger stone: Over 50 carats", Curiel said.
He pointed out that the cut is a "classical so-called emerald cut", which stands out from the typical, more rounded, multi-facetted cuts used today.
The auction house had expected to fetch $30 million to $50 million for the almost 19-carat, rectangular-cut stone, the largest fancy vivid pink diamond that it has ever put under the hammer.
Before the auction, an global jewellery specialist at Christie's, Jean-Marc Lunel, called it 'probably the most lovely (specimen) ever presented at public auction'.
All told, the diamond went for $US50.375 million, including the fees.
On Wednesday, Sotheby's will auction jewellery once owned by French Queen Marie Antoinette that had not been seen in public for 200 years.