See for Yourself One of the World’s Rarest Red Diamonds at the Smithsonian’s Natural History Museum
Unraveling the surprising science that gives colorful diamonds their special allure
Laid out like an orchestra in a quiet gallery room, 40 colorful gems have one of the planet’s most distinguished conductors: a red diamond, 2.33 of the rarest carats in the world, shining like a little bit of magic.
It’s called the Winston Red Diamond, and what makes it so exceptional is the nature of its redness. Designated as a “fancy red” by the Gemological Institute of America (GIA), a global authority on gems, it’s one of the largest of its kind ever found. (The Moussaieff Red Diamond is the largest in the public record at 5.11 carats.) The stone takes its name from Ronald Winston, who gave it to the Smithsonian’s National Museum of Natural History in 2023, continuing a tradition that began with his father, Harry Winston, the famous jeweler known as the “King of Diamonds,” who donated the Hope Diamond to the Smithsonian in 1958.
In the simplest terms, diamonds are carbon with a cubic atomic arrangement, and their shine comes from the gems’ high refractive index—that is, an ability to bend light. Diamonds are full of “fire,” as gemologists like to put it.
“There are very few red diamonds on the planet,” says Gabriela Farfan, a mineralogist and curator at the museum. And fancy red is a very specific distinction. “It’s not reddish brown, it’s not reddish purple, it’s not reddish pink. It’s red.”
Up close, the old-mine-cut diamond—shaped like a square, with rounded edges—has a stunning, saturated hue. Only about 30 percent of natural diamonds are considered gem-worthy, and among those, less than 1 percent are fancy color diamonds—that is, diamonds of unusual color. While the latter come in all hues, red seems to be the rarest. Fewer than 30 or so fancy red diamonds larger than one carat exist in the public record, Farfan says.
The Winston is the largest one going on public display. Beginning April 1, it will be featured alongside dozens of diamonds of many colors in a new ongoing exhibition, “The Winston Red Diamond and Winston Fancy Color Diamond Collection,” running indefinitely at the Natural History Museum’s Harry Winston Gallery.
Ahead of the exhibition, Farfan invited seven experts—from the Smithsonian, the GIA and the Paris School of Mines—to examine the diamond, starting last June. Farfan and her colleagues studied the diamond for more than six months, culminating in a paper that is set to be featured as the spring cover story in Gems & Gemology, the peer-reviewed journal published by the GIA.
Wuyi Wang, vice president of research and development at the GIA, helped facilitate the study of the Winston Red and emphasizes its significance. “A natural diamond getting a fancy red color grade is extremely rare,” he says. “First, it needs to be a perfect color in that range, and then the saturation needs to be perfect as well. If it’s too dark, it goes to black, and if it’s too light, it goes to pink.”
Because so few fancy red diamonds are available to researchers, Farfan says, “This will probably be the most detailed study of a fancy red diamond.” While the researchers had a lot of questions—about the stone’s origin, for example, and whose hands it has passed through—perhaps the most scientifically pressing is how and why fancy reds come to be. In their pure form, diamonds should be colorless, as carbon doesn’t give off any color by itself, says Eloïse Gaillou, director of the Mineralogy Museum at the Paris School of Mines, and one of the researchers who studied the Winston Red.
But sometimes a carbon atom is missing inside the diamond structure. That absence alone can produce some color because the structure is disturbed, and light interacts with the material differently. Other times the color comes from the replacement of a carbon atom by another element in the environment of the diamond when it grows. For example, gemologists attribute the color of fancy yellow diamonds to nitrogen impurities in the diamond structure. And for fancy blues, they credit boron.
Rather than an additional element, the crimson of a diamond like the Winston Red is believed to be caused by plastic deformation—that is, a permanent change in a material after undergoing applied stress. In the case of the Winston, that stress was the high temperature and heavy pressure it experienced deep underground.
In Gaillou’s view, our enduring fascination with diamonds begins with their births inside the Earth’s mantle. Because a diamond is formed in places humans can’t go, miles and miles and miles beneath the surface, she says, “It’s the only true messenger of the deep Earth.”
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This article is a selection from the April/May 2025 issue of Smithsonian magazine