This historic blue diamond is expected to fetch up to $5.3 million

A 6.16-carat blue diamond that was secretly passed down through European royalty over three centuries is going on sale for the first time in May.

The pear-shaped, dark grey-blue gem, known as The Farnese Blue, is believed to be one of the most important historical diamonds left in private hands, Sotheby's said.

Expected to fetch between $3.7 and $5.3 million when it goes under the auction house's hammer on May 15, in Geneva, the stone was found at India's Golconda mines. It was then given to Elisabeth Farnese, daughter of the Duke of Parma, in 1715 to mark her marriage to King Philip V of Spain.

Philip V and Elisabeth in 1739. (Image: Louis-Michel van Loo | Wikimedia Commons.)

The royal couple descendants owned the Farnese Blue, as they married into other European families, travelling from Spain to France, Italy and Austria. During more than 300 years, nobody - except from close relatives and the family jewellers - knew of the gem's existence.

The 6.16-carat, pear-shaped, dark grey-blue gem was found the Golconda diamond mines of India and was given to Elisabeth Farnese in 1715 to mark her marriage to King Philip V of Spain. (Image courtesy of Sotheby's.)

"All this time, it was hidden away in a royal jewellery box," Philipp Herzog von Wurttemberg, chairman of Sotheby's Europe, said in the statement.

"With its incredible pedigree, the Farnese Blue ranks among the most important historic diamonds in the world," he noted.

One of Elisabeth's descendants is the current owner of The Farnese Blue to be auctioned as part of Sotheby's Magnificent Jewel and Noble Jewels sale.

This is the first time The Farnese Blue, passed down through European royalty, goes on sale. (Image courtesy of Sotheby's.)