The 35-carat pear-shaped diamond Marie de Medici wore at her coronation in 1610, one of the world's most famous gems, is to be auctioned in Geneva on May 15, Sotheby's announced Tuesday.
Passed down through the royal families of France, England, Prussia and the Netherlands, the Beau de Sancy has witnessed 400 years of European history.
"The Beau Sancy is one of the most fascinating and romantic gems ever to appear at auction," David Bennett, from the auction house's jewellery department, said in a statement.
The stone -- which is expected to fetch $2-4 million -- gets its name from diamond collector Nicolas Harley de Sancy, who bought it in Constantinople, now Istanbul, in the late 16th century.
It is believed to have come from the city of Golconda, in central India, where other famous diamonds such as the Kohinoor and the Regent originated.
The 34.98-carat diamond measures 2.3 centimetres in height, is 1.9 cm wide and 1.1 cm deep.
Marie de Medici wore it mounted atop her crown for her coronation on 13 May 1610, the day before her husband, France's King Henry IV, was assassinated.
The Beau Sancy, which has rarely been shown to the public in recent decades, will go on a world tour from March and will be exhibited in Hong Kong, New York, Rome, Paris, London and Zurich before being sold in Geneva.
According to Sotheby's, when the last German Emperor and King of Prussia fled to exile in Holland in 1918, the crown jewels -- including the Beau Sancy -- remained at the Kaiser's palace in Berlin.
At the end of World War II, the collection was transferred to a bricked-up crypt in Bueckeburg, where it was later found by British troops. It was returned to the House of Prussia, which is now auctioning it.