Dubbed the ‘Farnese blue’, it was originally owned by Elizabeth Farnese, daughter of the Duke of Parma, who was given the jewel as a wedding gift in 1715.
It was later handed down through the generations, moving between Spain, France, Italy and Austria, but always kept as a well-guarded royal secret.
Extracted from the famous Golconda mine of India, the 6.1 carat trinket was the star attraction at the Sotheby’s auction.
Sotheby’s jewel specialist, Daniela Moschetti, said: “We were expecting a good result but we started from $3.5 million and we ended up with $6.7 million, so we exceeded our expectation.
“Good jewels, well-designed, well-made, with a signature, with a perfect slot in time, in age, do very well.”
Last month another Sotheby’s auction saw the sale of a rare blue diamond ring, also for $6.7 million.