A huge, rare deep-blue diamond known as the Wittelsbach-Graff Diamond will join the other gems at the National Museum of Natural History for seven months next year. The diamond, which weighs 31.06 carats, has not been on public display since an exhibition in Brussels in 1958.

The museum announced Monday that the current owner, Laurence Graff, chairman of Graff Diamonds International, is lending the brilliant gem not only for exhibition but also research. Graff bought the diamond last December at auction for $24.3 million, a record price for a single stone at the time.

The Wittelsbach-Graff Diamond is believed to be from the same mine in India as the world famous Hope Diamond. The Hope, which weighs 45.52 carats, has been one of the most popular attractions at the Smithsonian Institution for 50 years. The Smithsonian gemologists, under Jeffrey Post, curator of the National Gem Collection, plan to conduct experiments to see whether the smaller stone has the same properties as the Hope.

Besides its origin, the blue diamond has an intriguing past. In 1664 it was given to Infanta Margarita Teresa by her father, Philip IV of Spain, when she married Emperor Leopold I of Austria. Then it was owned by the House of Wittelsbach, a Bavaria ruling family, until their jewels were sold at Christie's in 1931.  The gemstone will be on display from Jan. 28, 2010, to Aug. 1, 2010.