The Gemological Institute of America (GIA) recently evaluated the Winston Red diamond – a very rare 2.33-carat red diamond now on display at the Smithsonian’s National Museum of Natural History in Washington, DC. 

The Winston Red is renowned as the fifth-largest diamond with an exceptionally pure red colour known to exist and the only such red diamond on public exhibit, the institute said. 

“The Winston Red diamond is one of the most exquisite gems on earth, from its unparalleled deep-red colour to its rich history,” said Susan Jacques, GIA president and CEO. “Natural fancy colour diamonds are very scarce, and red diamonds are exceedingly rare treasures of Mother Nature. Just two dozen pure red diamonds over one carat exist in the public record.”

GIA collaborated with the Smithsonian National Museum of Natural History Department of Mineral Sciences and the curator from the Paris School of Mines to examine the diamond.

Using advanced instrumentation and analysis, research and historical records, GIA determined the cause of the diamond’s rare colour and its possible geographic origin, which is most likely Brazil or Venezuela, based on its mineralogical characteristics and history.

Tom Moses, GIA executive vice president and chief laboratory and research officer, said he recalls examining the Winston Red in 1987, adding that “it is an unforgettable diamond.” 

He also described it as historic, adding that its old mine cut, deep-red hue and inclusions tell a story that can be traced back to September 1938 when Jacques Cartier sold the stone to the Indian Maharaja of Nawanagar.

Meanwhile, Dr. Ulrika D’Haenens-Johansson, senior manager of diamond research at GIA in New York City, said the red hue can be attributed to features introduced by plastic deformation – subtle changes to the diamond’s crystal structure caused by a long and stressful history under tremendous heat and pressure deep within the earth. 

According to GIA, only one in 10,000 diamonds has a noticeable fancy colour outside of the subtle colour range normally associated with colourless diamonds. Of the more than one million colour diamonds examined by GIA, only 0.07 per cent are red, and just over half of those received the coveted GIA colour grade of ‘Fancy red’ given to the Winston Red.

The Winston Red is the centrepiece of a new exhibit of natural fancy colour diamonds that opened on April 1, 2025 at the Smithsonian’s National Museum of Natural History. The exhibit features 40 other gems from the Winston Fancy Color Diamond Collection, gifted to the museum by Ronald Winston, the son of distinguished jeweller and gem collector Harry Winston.