Glittering atop a succession of royal crowns, the 35-carat "Beau Sancy" diamond has been witness to 400 years of European history.
Now the jewel, passed down through the royalty of France, England, the Netherlands and Prussia, could leave its noble past behind when it is sold at auction in Geneva next week.
The gem is being auctioned at one of three multi-million dollar sales over as many days in the city, featuring the jewels of queens, film stars and billionaires alike.
"The Beau Sancy is one of the most fascinating and romantic gems ever to appear at auction," said David Bennett of sellers Sotheby's, who estimates its value at $2 million-$4 million (1.5 million-3 million euros).
"One client I showed it to was moved to tears by it," he said.
The pear-shaped diamond's royal connections date back to 1604 when it was bought for Henri IV of France at the insistence of his wife Marie de Medici who wore it atop her crown at her coronation.
Later that century it was acquired by the Dutch and used to seal the wedding of Willem II of Orange Nassau to Mary Stuart, daughter of Charles I of England.
Stuart pawned the rose-cut gem to finance her brother Charles II's fight for the throne.
In 1702 the first king of Prussia gave it pride of place in the new royal crown and it has passed through generations of the House of Prussia until today.
"We've sold much larger diamonds but it has this wonderful romantic history, an unparalleled royal history -- it has never been in non-royal hands," said Bennett.
The Beau Sancy will go under the hammer on May 15 as part of Sotheby's "Magnificent Jewels and Noble Jewels" sale, also featuring a historic yellow diamond once the property of Charles Edward Stuart, one-time pretender to the thrones of Great Britain and Ireland.
More commonly known as "Bonnie Prince Charlie", his attempts to make the Stuarts regain the crown failed and following the Battle of Culloden in 1745 he went into exile in France and Italy, where he is thought to have offered the 7.3-carat gem to the Corsini family in gratitude of their support.
It has an estimated value of $300,000 to $500,000.
A collection of 70 jewels belonging to billionaire philanthropist Lily Safra is meanwhile expected to raise more than $20 million for charity when it is auctioned by Christie's on May 14.
Brazil-born Safra, 77, was married to the Jewish-Lebanese banker Edmond Safra who died in a blaze at his Monte-Carlo penthouse in 1999.
The Elton John AIDS foundation, a water treatment programme in Brazil and children's hospital in Israel are among 20 charitable institutes to benefit from the sale of the gems, many of them created specially for Safra by renowned Paris jeweller JAR.
A ruby-encrusted camelia brooch is expected to fetch up to $1.5 million and a pair of pear-shaped, 19-carat diamond earclips are worth an estimated $5 million.
On May 16, Christie's will host a "Sparkling Jewels" sale featuring a necklace set from the collection of Mexican screen actress Maria Felix (1914-2002) and a diamond once famously refused by Hollywood star and jewellery queen Elizabeth Taylor.
The 23.6-carat brown-orange coloured stone was offered to Taylor by Richard Burton in 1975 while the couple were in Africa, according to Christie's.
Taylor is said to have refused the gift, arguing that the money should rather be spent on building a hospital in Botswana.
The gem set into a ring is expected to fetch $600,000 to $800,000.
