The Hope Diamond that famously resides at the Smithsonian's National Museum of Natural History in Washington, D.C. has long been known for its inimitable color—a deep, steely blue, shifting ever-so-slightly in the light. It’s a hue that replica makers have tried and failed to copy; curator Jeffrey Post says the color attempt is always “garishly awful,” an aquamarine blue or a sickly “Windex blue.”
The true color is a trick of the light, thanks in part to the gemstone’s unique blue color and cut. It hasn’t always looked this way, either. When Jean Baptiste Tavernier first sold the original 112-carat diamond from India to King Louis XIV in 1668, it was crudely cut and a lighter color. Tavernier called it "un beau violet" (a beautiful violet). It would become bluer and darker as the gem passed through different hands, both French and American, and was recut twice more.
For the first time, scientists have created near-perfect cubic zirconia replicas of the diamond in its previous forms: the original brought from India, King Louis XIV’s “French Blue” and the current version encased in a Cartier pendant. It’s a project that’s taken a decade to perfect, involving cross-Atlantic collaborations between the Smithsonian, the Muséum National d’Histoire Naturelle in Paris and the gem-cutter John Hatleberg.
“We’ve been able to use a modern technology to bring back from history a diamond that no one has seen since 1792,” said Post at an October press conference. “We are actually putting ourselves back in the eyes of King Louis XIV and seeing what he saw.”
When the famous blue diamond first joined Louis XIV’s crown jewels in 1668, it was a lot bigger and flatter than it was today. It was cut in the Mughal style, with a large, flat base and top to match. Because there weren’t as many facets—the small flat faces on a crystal surface—to reflect light internally, it was a much lighter blue. “It’s like looking through a window,” Post said, holding the replica up to a lamp.
The gem became much darker and smaller once the court jeweler got his hands on it. It was cut with more facets and shrunk to 69 carats. It was then that it became known as the “French Blue,” said François Farges of the Muséum National d’Histoire Naturelle, and it perfectly matched Louis XIV’s sensibility.
“Think of the garden at the Castle of Versailles with the straight lines, perfectly well-arranged symmetry, good angles,” Farges said. “It is the same inspiration you have in the French Blue.”
It was cut precisely but unusually, with a small eye in the middle that let light pass clear through. This was done intentionally, as Farges and Post proposed in a 2014 paper. According to the crown jewels inventory, the diamond was set into gold and mounted on a stick. Farges found that, when placed in a gold setting via a computer model, the center of the diamond would look like a golden sun—the symbol of Louis XIV, “the sun king.” It’s even more impressive, Farges said, if you consider that the blue of the diamond and the gold of the sun represented the French monarchy.
Now, Post could hold up a replica of the French Blue placed in a facsimile of its gold setting and show the faint golden sun-like shape in the middle. Farges said that historical records suggest Louis XIV would have pulled the jewel from a gold chest and proudly displayed the stick for important visitors; it was meant to be observed, not worn.
“The big message was that France was so rich that they could use all those diamonds at any time to build a huge army in case the country would be invaded,” Farges said. “It was really a political instrument just to serve the glory of the king against the foreign kingdoms.”
All of these details about the diamond’s journey, color, faceting and use wouldn’t have been discovered without historical records like Tavierner’s drawings in his journal and those Farges has studied at the Muséum National d’Histoire Naturelle.
Farges said he stumbled upon one of the most important records about the diamond by accident. He was looking through the drawers of diamonds in the national gem collection and found a lead cast. He was puzzled by its shape, which didn’t resemble any type of diamond he had seen before, so he spent the night doing research. To his surprise, he found that it was the lead cast for the French Blue.
“I came to the conclusion that it was the only replica, the only historical replica known to date of a French blue diamond, that we thought was completely lost. It’s not totally lost,” Farges said. “For the first time, we had the model in 3D that was perfectly accurate, with all of the facetings.”
Before creating the older versions of the diamond, Hatleberg set out to make a replica of the current version of the Hope. Almost 30 years ago, Post took the Hope out of its pendant setting and gave it to Hatleberg so he could make a silicone mold and then a resin epoxy cast. From there, he cut cubic zirconia to match the cast, and then brought several of the copies to a company in Minnesota to add the coloring.
To replicate the color, the company used a method called precious metal nanodot vapor deposition. They take a colorless stone and thinly coat it with metal atoms, making tiny adjustments to ever-so-slightly tweak the color. This technology wasn’t available even five years ago, Hatleberg said.
Hatleberg would then come to Post and Farges with copies coated with different colors. “We’d go, ‘A little too dark, too light, too green, too blue, too purple,’” Post said. “And after literally years of doing that, dozens of trips back and forth, we finally ended up with a stone that all of us here, all of us who know the Hope Diamond, looked at and said, ‘We can’t tell the difference.’”
The replica might look exactly the same as the original Hope Diamond, but aspiring jewel thieves or counterfeiters, beware; there’s no way that you could slip a fake past an expert. Under an ultraviolet light in a dark room, the Hope Diamond phosphoresces, Post says, glowing orange for about a minute or so. He can use a spectrometer to measure the light spectrum, which differs from diamond to diamond like a fingerprint, he says.
It’s unclear when visitors will be able to look at the replicas in real life, but Post says he hopes the stones will be on display at the Natural History Museum within the next year or two. There will also be a set of replicas that travel around the country out on loan, and a set for the Muséum National d’Histoire Naturelle.
In the meantime, Post said, he hopes to study the Hope’s history even further with Hatleberg and Farges. “It’s such an interesting diamond, both scientifically and historically,” Post said. “We know we’re going to keep learning from it. We’ve only begun to learn all of its secrets.”