This diamond will be the largest fancy vivid blue diamond ever sold at auction and is a so-called rectangular cut — among the most prized in the diamond world. Its value is also enhanced by its previous owner. The diamond, called the “Oppenheimer Blue,” was owned by Philip Oppenheimer, the late chairman of the De Beers diamond-marketing cartel.

“Here’s a man who could have had any diamond in the world, and this is the blue diamond he chose to have,” said Rahul Kadakia, Christie’s head of jewellery. “This is the ‘Mona Lisa’ of blue stones.”

What is so special about blue diamonds?

In the diamond world, blue is one of the rarest and most sought-after colours. Kadakia said that less than 2 percent of diamonds discovered in the world are blue (the colour comes from the presence of boron in the diamond). Only 10 percent of all blue diamonds discovered are larger than a carat — so the Oppenheimer Blue is a highly prized freak of nature. It also has the highest colour rating of “vivid.”

“Blue diamonds range from a very faint baby blue, a powder blue, to a dark, dark-indigo blue [that’s] almost too dark,” Kadakia said. “Vivid is the purest, most open, almost a happy blue colour. It’s dark, but not too dark and not too light. It’s sort of a very rich, navy blue.”

Blue is also a popular diamond colour because it can be worn by both men and women. Kadakia said men in the Middle East are especially fond of wearing blue diamond rings.

“A lot of our big collectors in the Middle East, who do wear diamond rings for specific reasons, will wear a blue diamond,” he said. “And not so much a pink or a green or a yellow diamond.”

Blue diamonds are also the most expensive on a per-carat price. The 12.03-carat “Blue Moon Diamond” was sold by Sotheby’s in November for $48.5 million, setting a new record of $4 million per carat. If the Oppenheimer sells for the same per-carat price it would fetch more than $58 million. Christie’s official estimate on the stone is more conservative, though experts say it’s likely to sell for more than the $45 million top estimate.

“It’s a bargain to start with,” Kadakia said. “We’ll see what happens.”

The stone will be sold at Christie’s Geneva Magnificent Jewels sale on May 18.