Scientists Create First Hexagonal Diamond Harder Than Natural Diamond

For nearly sixty years, hexagonal diamond has occupied one of the most contested corners of materials science — a substance first glimpsed in the wreckage of a meteorite impact, theoretically predicted to outperform the hardest known natural material, and yet so elusive that a prominent 2014 paper declared it did not exist at all. That debate appears to be over.

A team of Chinese physicists led by Chongxin Shan at Zhengzhou University has synthesized pure, millimeter-sized crystals of hexagonal diamond and measured their properties for the first time, confirming the material is stiffer, more oxidation-resistant, and slightly harder than conventional cubic diamond [1][2]. The results, published in Nature in March 2026, represent what independent experts are calling the strongest evidence yet that this long-disputed carbon allotrope is real — and could eventually reshape industries that depend on the hardest materials on Earth [3].

What Is Hexagonal Diamond?

Diamond, the substance most people picture when they think of "the hardest material," arranges its carbon atoms in a cubic crystal lattice — a repeating three-dimensional pattern of interlocking cubes. Hexagonal diamond organizes the same carbon atoms in a lattice made of hexagons, like a honeycomb. The difference is subtle at the atomic level but has profound implications for the material's mechanical properties [4].

The hexagonal variant is formally known as lonsdaleite, named after Dame Kathleen Lonsdale, the pioneering Irish-British crystallographer who proved the flat structure of the benzene ring in 1929 using X-ray diffraction and became one of the first two women elected as a Fellow of the Royal Society in 1945 [5]. The mineral was first identified in 1967 in fragments of the Canyon Diablo meteorite at Arizona's Meteor Crater, where the extreme heat and pressure of the impact had transformed graphite into diamond while preserving graphite's hexagonal crystal structure [4].

Theoretical calculations have long predicted that hexagonal diamond should be harder than its cubic cousin — some simulations suggested up to 58% harder. But until now, nobody had produced samples pure enough and large enough to test those predictions rigorously.

The Breakthrough: Crushing Graphite Into a New Diamond

Shan's team started with highly oriented pyrolytic graphite — an ultrapure, highly ordered form of the material — and squeezed it between anvils made of tungsten carbide under 20 gigapascals of pressure, roughly 200,000 times atmospheric pressure, while heating it to temperatures between 1,300°C and 1,900°C (approximately 2,370°F to 3,450°F) [2][3].

The result: millimeter-sized samples of hexagonal diamond, confirmed through X-ray diffraction and atomic-scale electron microscopy. This is orders of magnitude larger than previous claimed samples, which were typically nanometers across and too small to characterize definitively [6].

The hardness measurements told a nuanced story. At approximately 114 gigapascals on the Vickers hardness scale, hexagonal diamond proved harder than the typical natural cubic diamond at roughly 110 GPa — but not the 58% improvement that some theoretical models had predicted [7]. The material did, however, demonstrate superior stiffness and notably better resistance to oxidation, beginning to oxidize at 848°C (1,558°F), significantly higher than cubic diamond [1].

"These findings resolve the long-standing controversy on the existence of hexagonal diamond as a discrete carbon phase," the researchers wrote [6].

A Six-Decade Scientific Feud

The achievement is as much about ending a bitter scientific argument as it is about creating a new material. The history of lonsdaleite research reads like a whodunit in which the central question — does the suspect even exist? — remained unresolved for decades.

After the 1967 meteorite discovery, laboratories around the world attempted to synthesize hexagonal diamond. Hundreds of claims followed [3]. But the samples were consistently tiny, contaminated, or ambiguous. The fundamental problem was that defective cubic diamond can produce X-ray diffraction patterns that closely mimic those expected from a true hexagonal structure. Without samples large and pure enough for definitive analysis, it was impossible to distinguish the real thing from an impostor.

The skeptics had their moment in 2014, when a team led by Péter Németh published a landmark paper in Nature Communications titled "Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material" [8]. Using ultrahigh-resolution electron microscopy, Németh's group argued that every sample previously attributed to lonsdaleite was actually ordinary cubic diamond riddled with structural defects — twins and stacking faults that produced misleading diffraction signatures.

The paper was a bombshell. If Németh was right, decades of lonsdaleite research had been chasing a phantom.

But the counterattack came. Studies by Kraus et al. in 2016 and Turneaure et al. in 2017 demonstrated lonsdaleite formation through shock compression of graphite, suggesting it could indeed form as a separate species [4]. In 2025, physicist Ho-kwang Mao, director of the Shanghai Advanced Research in Physical Sciences centre, led an independent team that confirmed the synthesis of hexagonal diamond, providing crucial corroboration ahead of Shan's definitive work [3].

Oliver Tschauner, a mineralogical crystallographer at the University of Nevada, Las Vegas, who served as a peer reviewer for the new study, put it plainly: "There are hundreds of claims from people who believe they have seen it," but characterized Shan's work as "the first very accurate characterization of this elusive material" [3].

Why It Matters: Beyond Bragging Rights

The practical implications extend well beyond settling an academic dispute. The global synthetic diamond market — already valued at roughly $25 billion in 2025 and projected to reach $38 billion by 2031 — is driven by industrial demand for cutting tools, drilling equipment, abrasive coatings, thermal management materials, and increasingly, components for advanced electronics [9].

Cubic diamond, for all its hardness, has a critical weakness: it begins to oxidize at relatively low temperatures, limiting its usefulness in high-heat industrial applications. Hexagonal diamond's superior oxidation resistance could make it significantly more durable in environments where conventional diamond tools degrade — deep-earth drilling, high-speed machining of hard metals, and thermal management for high-power electronics [7].

Shan himself pointed to three areas of potential application: "cutting tools, thermal management materials, and quantum sensing" [7]. The quantum sensing angle is particularly intriguing — diamond-based quantum sensors are already used in precision measurement applications, and a harder, more thermally stable variant could expand their operational envelope.

Source: Mordor Intelligence

Data as of Mar 17, 2026CSV

The Caveats

Independent experts urge caution about how quickly hexagonal diamond could move from laboratory curiosity to industrial product. Several significant hurdles remain.

First, the synthesis conditions are extreme. Generating 20 gigapascals of pressure at temperatures exceeding 1,300°C requires specialized equipment that is expensive and difficult to scale. Current synthetic diamond production relies on high-pressure, high-temperature (HPHT) methods and chemical vapor deposition (CVD), both of which operate at far lower pressures [9].

Second, the hardness advantage is modest. At 114 GPa versus 110 GPa, hexagonal diamond is roughly 3-4% harder than typical cubic diamond — significant in materials science but not the transformative leap that some theoretical predictions promised. The real competitive advantages may lie in oxidation resistance and stiffness rather than raw hardness.

Third, the samples remain small. A millimeter-wide crystal is a giant leap from nanometer-scale fragments, but it is still far from the sizes needed for most industrial tools. Scaling production to commercially viable dimensions will require significant engineering advances.

The Bigger Picture: Carbon's Infinite Versatility

The creation of hexagonal diamond adds another chapter to carbon's extraordinary story. The same element that forms the graphite in a pencil lead also produces the hardest known natural material — and now, something harder still. Carbon already exists in a dazzling array of allotropes: cubic diamond, graphite, graphene, fullerenes (buckyballs), carbon nanotubes, and amorphous carbon, each with radically different properties determined solely by atomic arrangement.

The successful synthesis of hexagonal diamond reinforces a principle that has driven materials science for decades: the properties of matter are determined not just by composition but by structure. The same atoms, arranged differently, can produce materials softer than talc or harder than anything found in nature.

The achievement also underscores the growing dominance of Chinese research institutions in high-pressure physics and materials science. Zhengzhou University, while less internationally prominent than institutions like Tsinghua or Peking University, has built significant capabilities in diamond synthesis and high-pressure research — a reflection of China's strategic investment in advanced materials, which Beijing's 15th Five-Year Plan, approved earlier this month, identifies as a priority sector for technological self-reliance [10].

What Comes Next

The immediate scientific priority will be independent replication. While Mao's 2025 work provides strong corroboration, the extraordinary nature of the claim — resolving a six-decade debate — demands rigorous verification by multiple independent groups using different synthesis methods.

If the results hold, the race will shift to scaling production. The synthetic diamond industry has proven remarkably adept at turning laboratory novelties into commercial products — today's lab-grown diamonds, virtually indistinguishable from natural ones, were once considered impossible to produce at scale. Hexagonal diamond may follow a similar trajectory, though the extreme synthesis conditions present a steeper engineering challenge.

For now, the achievement stands as a testament to patient, meticulous science. Nearly sixty years after a meteorite slammed into the Arizona desert and left behind microscopic traces of a mysterious material, researchers have finally proven what that material is — and shown that humans can make it too.