regent diamond

 A famous diamond of fine quality, which weighs about 137 carats and is among the state jewels of France. It is so called from the Duke of Orleans, Regent of France, to whom it was sold in 1717 by Pitt the English Governor of Madras (whence also called the Pitt diamond), who bought it of an Indian merchant in 1701.  [Webster 1913 Suppl.]