A large pink diamond a could become one of the most expensive gems ever sold at auction, estimated to fetch up to $50 million next month in Geneva.

The 18.95 carat, fancy vivid pink stone is a rectangular cut and is called The Pink Legacy. Christie's will put it up for auction Nov. 13 at its Magnificent Jewels Sale in Geneva, with an estimate sale price of $30 million to $50 million, according to Christie's.

Fancy vivid pink diamonds are extremely rare, especially those more than a carat or two. Only four over 10 carats in size have ever come up for auction in 250 years, Christie's said. The Pink Legacy also has a type IIA classification, meaning it is among the most chemically pure diamonds.

The gem traces its history to the Oppenheimer family, which adds to its worth. The famed South African diamond dynasty controlled much of the world's trade over three generations. Nicky Oppenheimer sold the family's 40 percent stake in diamond mining company De Beers Group for $5.1 billion to Anglo American in 2011, after taking De Beers private in 2001.

"The discovery of this previously unrecorded and remarkable diamond will cause immense excitement with collectors and connoisseurs of diamonds around the world," said Rahul Kadakia, international head of jewelry for Christie's.

The Pink Legacy won't be the most expensive stone ever sold. That honor is still held by The Pink Star, a 59.60 carat fancy vivid pink diamond that sold in Hong Kong by Sotheby's last year for $71.2 million. It first sold at auction in 2013, when Sotheby's sold it for $83 million to a New York-based diamond cutter. But the buyer defaulted on the deal, so it came back up for auction.

The second most expensive diamond ever sold is the Oppenheimer Blue diamond, which had also been in the Oppenheimer family, sold for $57.2 million in 2016. If the Pink Legacy fetches $50 million, it would become the third most expensive ever auctioned.