WASHINGTON -- Light flashes across the 82 facets of the Tiffany Diamond, highlighting the brilliance of the giant gem at the Smithsonian's National Museum of Natural History.

One of the world's largest yellow diamonds, the stone is on loan from Tiffany & Co. through Sept. 23.

It joins such famed jewels as the Hope Diamond, Hooker Emerald and Oppenheimer Diamond.

The Tiffany Diamond weighs 128.54 carats and is in a cushion cut. Perched on it is a gem-encrusted bird known as the "Bird on a Rock," designed in the early 1960s by Jean Schlumberger.

The bird is gold and platinum with white and yellow diamonds accented by a ruby eye.

"It's the largest diamond on public display in the United States," said Jeffrey E. Post, curator of gems at the museum.

It is more than 2 1/2 times the size of the famed Hope Diamond, which weighs in at 45.5 carats.

"It's on summer vacation," Fernanda M. Kellogg, president of the Tiffany & Co. Foundation, said of the loan. The stone is priceless, she said.

It has only been worn twice, once by a Rhode Island socialite and once by actress Audrey Hepburn in a promotion for the film Breakfast at Tiffany's, said Linda Buckley, a Tiffany vice president.

Post said displaying the gem is part of a celebration of the foundation's endowment to the museum, which allows it to purchase gems to fill in its collection.

The Tiffany Diamond's 82 facets are 24 more than in a traditional brilliant-cut diamond, a step intended to maximize the sparkle -- called fire in diamonds.

Discovered in South Africa in 1877, the stone was purchased by New York jeweler Charles Tiffany.

His gemologist, George Frederick Kunz, studied the gem for a year before beginning to cut it -- reducing it from 287 carats to its current size.

Also joining the museum collection through Tiffany's endowment are a heart-shaped, dark-green garnet found near the Kenya-Tanzania border and a square purple tourmaline from Mozambique.