After five minutes of bidding, the 18.96-carat Pink Legacy became the world record holder for price paid per carat for a pink diamond at auction.
The "Pink Legacy" diamond sale is a new world record price per carat for a pink diamond.
The price of around $2.6m per carat marked a world record for a pink diamond, according to the Europe head of auction house Christie's.
Graded "vivid", the highest rating for a pink diamond's color, the gem is internally pure with a rectangular cut, and mounted on a platinum ring.
Christie's global head of jewelry, Rahul Kadakia, said it was purchased by American luxury brand Harry Winston, and will be renamed the Winston Pink Legacy.
The rectangular-cut stone was found in a South African mine around a hundred years ago and has likely not been altered since it was first cut in 1920, Christie's said.
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He pointed out that the diamond has a "classical so-called emerald cut" - standing out from the typical cuts used today, which are more rounded and multi-faceted.
Fancy Vivid Pink diamonds over 10 carats are extremely rare.
"It is probably the most handsome (specimen) ever presented at public auction", Lunel said.
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 USA $30 to $50 million.
The Pink Legacy's record-smashing price accounted for almost half the takings at Tuesday's auction, which saw total sales of more than $110 million, according to Christie's spokeswoman Alexandra Kindermann.
All told, the diamond went for $50.375 million, including the fees. On Wednesday, Sotheby's will auction jewelry once owned by French Queen Marie Antoinette that hadn't been seen in public for 200 years.