The Farnese Blue – a 6.16-carat fancy dark grey-blue diamond which has subsequently passed down through four of the most important royal families in Europe: Spain, France, Italy and Austria – appeared on the market for the first time during May 2018. Having traveled around Europe for three centuries, the stone was hidden away in a royal jewelry box. Except for close relatives, and of course the family jewelers, no one knew about its existence.