At 10.95 carats, the stone is the largest triangular-shaped fancy vivid blue diamond ever to come to auction, Christie's told The Associated Press in advance of the Oct. 20 sale. It is paired with a 9.87-carat white diamond cut in the same shape.
"Vivid blue is the strongest and purest saturation in any colored diamond," said Rahul Kadakia, Christie's jewelry expert. "As a vivid, this is as good as it gets."
The two diamonds were cut to be together.
"They are perfectly matched in size and shape. They may be different in terms of weight, but the measurements are perfect. These two stones are made for each other," Kadakia said.
The Gemological Institute of America said the blue stone in Christie's sale is the largest triangular-shaped fancy vivid blue diamond they've ever graded, Kadakia said.
It is being sold by an anonymous European businessman. Kadakia described him as "someone with a very, very keen eye" who purchased the ring for $1 million from Italian luxury jeweler Bulgari in Rome in 1972 a gift to his wife for the birth of their son. The couple went on to have three more children, celebrating each birth with an important gift.
Bulgari's distinctive geometric forms, classic Greek and Roman influences and unusual combinations of colored and colorless diamonds have long adorned celebrities and movie stars, including Grace Kelly, Audrey Hepburn and Elizabeth Taylor.
Last year, a 7.03-carat cushion-cut blue diamond sold at Sotheby's for $9.5 million, or $1.3 million per carat setting the highest price ever for a fancy vivid blue gem.
Based on that sale, the Bulgari blue diamond alone is worth $15 million, said Kadakia. With the white diamond, the ring could well surpass that estimate.
The Wittelsbach-Graff Diamond, a 17th-century fancy deep grayish-blue 35.56-carat gem, holds the world record for any diamond and jewel sold at auction. Christie's sold it in 2008 for $24.3 million.
Worldwide, Kadakia said, blue diamond production accounts for 0.0001 percent of all diamonds produced. And only one in 10 million diamonds have a color pure enough to qualify as fancy vivid blue and measuring over 10 carats.
The blue is determined by trace amounts of boron. The GIA scale runs from faint blue, light blue, fancy light blue, fancy blue, fancy intense blue, deep blue and when the tonality and saturation are perfect fancy vivid blue.
The Bulgari blue diamond probably came from the Premier Diamond Mine in South Africa, virtually the only mine in the world producing blue diamonds, Kadakia said. It is where the 3,100-carat rough Cullinan Diamond was found in 1905, and from which the British Crown Jewels the Great Star of Africa and Cullinan II diamonds were cut and polished.
Kadakia estimated that the Bulgari blue diamond came from a rough of at least 20 carats the rest being lost to polishing.
The world's largest known deep blue diamond is the 45.52-carat Hope Diamond, discovered in the 1600s, and housed at the Smithsonian.
Kadakia said that in the last 2½ years he has seen a 15-20 percent rise in the number of clients investing in high-end jewelry and diamonds, both those who buy at auction and privately.
"In what other form could you carry $15 million to $20 million so easily?" he asked.
The ring will be officially unveiled at Christie's Geneva gallery on Tuesday, with stops later in Hong Kong and London before returning to New York.