The Collaborative International Dictionary of English v.0.48:

Pitt diamond \Pitt diamond\, n.
   See regent diamond.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Regent diamond \Re"gent di"a*mond\
   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.]