New claim to make elusive ‘hexagonal’ diamond is strongest yet

After decades of debate, researchers say they have found the clearest evidence yet of this rare form of carbon.

By Mark Peplow & Nature magazine

Diamond is known as the hardest mineral on Earth. But researchers are studying an unusual variation, known as the hexagonal diamond, that could be even more difficult. After decades of claims and counter-claims about the possibility of synthesizing this mysterious material in the laboratory, Chinese researchers report that they have succeeded.

Scientists covet this material because it “has potential applications in many areas, for example in cutting tools, in thermal management materials and in quantum detection“, explains Chongxin Shan, a physicist at Zhengzhou University, who co-led the work.

“There are hundreds of claims from people who believe they saw it,” says Oliver Tschauner, a mineralogical crystallographer at the University of Nevada, Las Vegas, who reviewed the paper. “But this is the first very precise characterization of this elusive material.”

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Shock value

Conventional diamond consists entirely of carbon atoms arranged in tetrahedra, which ultimately form a cubic crystal structure. Viewed from a specific angle, this network of atoms looks like a stacked series of curly honeycomb layers. Each successive layer is slightly offset from its neighbors, in a pattern that repeats every three layers. But in 1962, researchers predicted that diamond might adopt a different structure – with hexagonal features – in which the pattern would repeat every two layers.

In conventional or cubic diamond, the carbon bonds between layers are slightly weaker than those within layers, limiting the strength of the diamond. In the hexagonal shape, the bonds between layers are shorter and stronger than those in cubic diamond, and predictions suggest that these characteristics should make hexagonal diamond more than 50% harder.

In 1967, researchers reported discovering a hexagonal diamond in a meteorite found in Arizona, which was part of the space rock that created the iconic Meteor Crater nearby. The team suggested that the shock of the impact transformed the meteorite’s graphite into hexagonal diamond and named this new mineral lonsdaleite, in honor of pioneering crystallographer Kathleen Lonsdale.

Around the same time, another research team reported producing hexagonal diamond in the laboratory by heating and compressing graphite. But some scientists have cast doubt on this report. And others have argued that lonsdalite is not a hexagonal diamond at all; They said it was just a cubic diamond with several flaws.

Peak demand

Much of the debate stems from X-ray diffraction experiments used to discern the crystal structure of the material, Tschauner says. In this type of experiment, when X-rays scatter through a crystal, some of them combine and produce peaks in X-ray intensity that reveal the position of the atoms. However, the pattern of diffraction peaks obtained from a highly defective cubic diamond would closely mimic hexagonal diamond, Tschauner says. To conclusively demonstrate the hexagonal structure, a few additional telltale peaks must be present. “This new paper shows these peaks,” he says. “That’s why I believe it.”

Shan and his colleagues started with highly oriented pyrolytic graphite, then pressed it between tungsten carbide anvils under a pressure of 20 gigapascals (200,000 times atmospheric pressure) between 1,300 and 1,900 °C to produce millimeter-sized samples of hexagonal diamond. Tests showed the material to be stiffer, more resistant to oxidation and slightly harder than cubic diamond.

Last year, another research group independently reported making a hexagonal diamond. “It seems that the new paper is very similar to ours. I have to say that I don’t see any difference,” says Ho-kwang Mao, a physicist and director of the Shanghai Physical Sciences Advanced Research Center in China, who led the team involved in the 2025 paper. “But we are happy that they reproduced our results.”

Hexagonal signs

“It’s almost the same thing,” says Tschauner, pointing out that the X-ray analysis done by Mao and his colleagues lacked one or two of the diffraction peaks you’d expect to see in the hexagonal diamond. A third group also reported in 2025 that they had made a “nearly pure” hexagonal diamond that was harder than the cubic diamond.

Mao says that tiny traces of cubic diamond that contaminated samples produced by both his and Shan’s groups could explain why their hexagonal diamond is not as hard as expected. “If we can get rid of all this, we can probably make things even more difficult,” he says.

Taken together, these papers should be enough to convince hexagonal diamond skeptics that the material exists and can be made in a lab, Shan says.

This work could also reignite the search for true hexagonal diamonds in meteorites, Tschauner says, because it proves that the material can be created by pressures and temperatures consistent with meteor impacts. “I think we need to figure out if this actually exists in nature,” he says. “For meteorite research, the quest now becomes finding them.”

This article is reproduced with permission and has been published for the first time March 4, 2026.