The 400-year-old diamond, the Beau Sancy, is expected to sell for $4 million at an auction at Sotheby's, an aunction house. The Beau Sancy, which weighs 34.98 carats, is one of the world's oldest diamonds with a rare cut in the shape of a pear.

The jewel is currently on display in Paris, but will move to London and Zurich before it goes up for auction in Geneva on May 14. It is a colorless, rounded, pear-shape diamond, cut with 110 facets, according to Famous Diamonds.

It has a fascinating history, and really is a once-in-a-lifetime sale, said David Bennett, Sotheby's jewelry chairman, reported the Associated Press. When it was made in the 16th century, the pear cut was new, bold, and so it became the most sought-after jewel of its era. Before the 'Beau Sancy' all diamonds were rudimentary.

The diamond has a rich, noble history spanning many generations and royal kingdoms. In 1604, the diamond was purchased by King Henri IV of France from its first owner, Lord of Sancy. He bought as a gift for his wife, Marie de Medici, who was one of the richest women in Europe and part of the famed Medici family of Italy.

Unfortunately, Medici's husband was murdered and she lost all of her wealth. She was forced to sell the diamond to Frederick Henry, Prince of Orange, for 80,000 florins. The diamond passed through Henry's descendants and various royal families of Europe.

The Beau Sancy became one of the most prized stones in the Prussian Empire. The diamond even sat on the royal crown and was worn by women by of royal Prussian family during important occasions, reported the Jewellery Editor. 

When the last German Emperor and King of Prussia fled to exile in Holland, in November 1918, the crown jewels remained at the Kaiser's palace in Berlin, reported the Jewellery Editor.

At the end of World War II, the Prussian royal jewelry, including the Beau Sancy, was kept hidden in a crypt in Bückeburg until it was discovered by British troops. Subsequently, it was returned to the Prussians.

In the last 50 years, the jewel has only been show in public four times by the owner, the head of house of Hohenzollern, Prince Louis-Ferdinand of Prussia.