The Pink Legacy diamond has been estimated at between $30 million and $50 million (AFP)

The Pink Legacy, an exceptionally large pink diamond, is set to go under the hammer at Christie's in Geneva next week, when it is expected to bring in up to $50 million.

At nearly 19 carats, the vividly coloured gem is extraordinary, Jean-Marc Lunel, an international jewellery specialist at Christie's, told AFP.

"If you consider that most pink diamonds weigh less than a carat, it is really something," he said.

The gem, which on November 13 will be offered at auction for the first time, has been estimated at between $30 million and $50 million (26.5-44.1 million euros), and Lunel suggested it could hit the high end of that range.

"It is probably the most beautiful (specimen) ever presented at public auction," he said, insisting there was good reason to expect the rock to snatch an extraordinary price.

The rectangular-cut diamond has been graded "fancy vivid" -- the highest possible grade of colour intensity.

Christie's pointed out that in the salesroom, fancy vivid pink diamonds over 10 carats are "virtually unheard of" and that only four vivid pink diamonds or over 10 carats have ever been offered for sale at auction.

One of them, the nearly 15-carat Pink Promise, was sold last November at a Christie's auction in Hong Kong for $32.5 million. That amounts to $2.176 million per carat, which remains the world auction record price per carat for any pink diamond.

And in 2013, a huge pink diamond weighing 59.60 carats meanwhile went under the hammer at Sotheby's for $83 million, or $1.39 million per carat.

The Pink Legacy, set to headline Christie's annual Magnificent Jewels auction on Tuesday, used to belong to the Oppenheimer family, which for decades ran the De Beers diamond mining company, but Christie's refused to say who the current owner was.

It was discovered in a South African mine around a century ago, Lunel said.

"It was probably cut in the 1920s," he said, adding that it had not been altered since.

The classic rectangular cut is traditionally used for white stones, but is rare for pink diamonds.

"Imagine a domino that you have cut the corners off of," Lunel said, pointing out that the cut is a "classical so-called emerald cut", which stands out from the typical, more rounded, multi-facetted cuts used today.

Rahul Kadakia, Christie's international head of jewellery, meanwhile said in a statement in September that he expected The Pink Legacy to "cause immense excitement with collectors and connoisseurs of diamonds around the world".

"Its exceptional provenance will no doubt propel it into a class of its own as one of the world's greatest diamonds."