Pitt bought the diamond for £20,400 (£2,964,490 as of 2012), and had it cut in to a 141 carat cushion brilliant.After many attempts to sell it to various Members of European royalty, including Louis XIV of France, it was purchased by the French Regent, Philippe II, Duke of Orléans in 1717 for £135,000 (£18,634,090 as of 2012).The stone was set into the crown of Louis XV for his coronation in 1722, into a new crown for the coronation of Louis XVI in 1775, and used to adorn a hat belonging to Marie Antoinette. In 1791 its appraised value was £480,000 (£46,922,530 as of 2012).In 1792 during the revolutionary furor in Paris, “Le Régent,” as the diamond came to be known, was stolen along with other crown jewels of France, but was later recovered, after being hidden in some roof timbers in a Paris attic.

The diamond was used as security on several occasions by the Directoire and later the Consulat, before being permanently redeemed by Napoleon Bonaparte in 1801.
Napoleon used it to embellish his sword, designed by the goldsmiths Odiot, Boutet and Marie-Etienne Nitot.

In 1812, it appeared on the Emperor’s two-edged sword, the work of Nitot. Napoleon’s second wife, Archduchess Marie Louise of Austria, carried the Régent back to Austria upon his death. Later her father returned it to the French Crown Jewels. The diamond was mounted successively on the crowns of Louis XVIII, Charles X and Napoleon III.

Today, mounted in a Greek diadem designed for Empress Eugenie, it remains in the French Royal Treasury at the Louvre.

It has been on display there since 1887.

Sources:
Wikipedia
Famous Diamonds