Peter Buck, a nuclear scientist who made millions as co-founder of the Subway sandwich chain, has given the Smithsonian's National Museum of Natural History a rare 23.1-carat ruby in memory of his late wife.

The stone, named the Carmen Lucia ruby, is a brilliant red gem that was mined in Burma in the 1930s. It is set in platinum with two triangular colorless diamonds, measuring 1.1 and 1.27 carats, nudging the oval ruby.

It is one of the largest faceted Burmese rubies known to exist, says Jeffrey Post, curator of the National Gem and Mineral Collection. He said it is Smithsonian policy not to discuss the monetary value or sale price of its gems.

The rock caught light and shadows like a campfire as he held it under various lamps. "It is a stone with great clarity. It has a little bit of natural markings and defects that are characteristic of Burma," Post said. As he stood in the museum's vault and moved the stone, it also picked up a pinkish tone.

For the museum's chief geologist, the acquisition was as dazzling as the gem. "In 20 years this is the most important gift given to us," Post said.

It goes on display Saturday, joining the 45.52-carat Hope Diamond, the 275-carat Napoleon I Diamond Necklace, Marie-Antoinette's diamond earrings, the 423-carat Logan sapphire and more than 600 other gem and jewelry samples in the galleries. The National Gem Collection has more than 10,000 items.

"Nature doesn't grow large gem-quality ruby crystals. They are so rare," Post said. "It is the largest and finest faceted ruby on display anywhere in the world." Gem experts are not sure what exists in the vaults of the world's royal families.

The gift also comes with a story of love and success.

In 1965, Peter Buck gave his friend, Fred DeLuca, about $1,000 to open a submarine sandwich shop. DeLuca wanted to earn money for college and opened the store in Bridgeport, Conn. At first the venture was called Pete's Super Submarines, and later was shortened to Subway. Now there are more than 21,000 stores around the world -- including three at the Smithsonian -- making Subway the world's largest submarine sandwich chain. Buck, 73, is worth $1.5 billion and ranks No. 165 on Forbes's last count of America's richest.

After the ruby was mined, it was owned by two or three European families and then was purchased by a group of jewelers, Post said. A loose stone at this point, it was kept in a vault for 16 years.

When the group decided to sell the ruby, one of the partners brought it from New York for Post to see. Post wanted it for the museum's collection, but he knew it would be hard to find a donor to buy and then donate such a prize.

Carmen Buck, who was born in Brazil, saw a photograph of the stone after her jeweler learned it was coming on the market. After her death from cancer in 2003, her jeweler and her husband discussed the stone's importance. Then they called Post.

"Dr. Buck said his wife would like the idea of giving the gift to the United States. She was proud of her U.S. citizenship and she would have liked giving something back to the United States. He also said she would have liked that it would be on display for children for generations," said Post.

Peter Buck donated the 23-carat Burma ruby.