After 60 Years of Doubt, Scientists Forge the 'Impossible' Hexagonal Diamond — and It's Harder Than the Real Thing

A team of Chinese physicists has achieved what many materials scientists believed was impossible: synthesizing bulk, pure hexagonal diamond in the laboratory. The millimeter-sized crystals — a form of carbon that arranges its atoms in a hexagonal lattice rather than the familiar cubic structure of conventional diamond — are measurably harder, stiffer, and more oxidation-resistant than natural diamond [1][2]. Published in Nature in March 2026, the work settles a bitter scientific debate that has raged since the material was first reported in a meteorite crater nearly 60 years ago [3].

A Diamond Born From the Stars

Hexagonal diamond was first identified in 1967 in fragments of the Canyon Diablo meteorite, which carved Arizona's famous Meteor Crater roughly 50,000 years ago. Researchers proposed that the extreme heat and pressure of impact had transformed graphite within the meteorite into a diamond-like structure, but one that retained graphite's hexagonal crystal symmetry instead of converting fully to cubic diamond [4].

They named the new mineral lonsdaleite, after Dame Kathleen Lonsdale, the pioneering Irish-British crystallographer who became the first woman elected as a Fellow of the Royal Society. Lonsdale had established the structure of benzene through X-ray diffraction in 1929 — work that laid foundations for understanding how carbon atoms bond [4].

From the beginning, lonsdaleite tantalized physicists. Theoretical models predicted that its hexagonal lattice would produce shorter, stronger bonds between carbon layers, potentially making it up to 58% harder than cubic diamond — the hardest natural material known [5]. If true, hexagonal diamond would represent the ultimate superhard material.

Six Decades of Controversy

But the tantalizing predictions ran into a wall of skepticism. Every time researchers claimed to have found or created lonsdaleite, critics raised the same objection: the samples were too small, too impure, or too ambiguous to rule out an alternative explanation.

The most damaging blow came in 2014, when a team published a study in Nature Communications arguing that lonsdaleite "does not exist as a discrete material" [6]. Using ultrahigh-resolution electron microscopy, they showed that samples previously identified as hexagonal diamond were actually cubic diamond riddled with structural defects — twins and stacking faults that mimicked the diffraction pattern of a hexagonal crystal. It was, they argued, an artifact, not a new form of carbon.

"Every time scientists thought they had found lonsdaleite, skeptics asked the same question: was this really a distinct crystal, or just ordinary cubic diamond full of defects?" as Nature summarized the decades of debate [3].

The controversy left the field polarized. Hundreds of claims were made over the years, but none produced samples large enough or pure enough to withstand scrutiny.

The Breakthrough: Crushing Graphite Into History

The new work, led by Shoulong Lai and co-senior author Chongxin Shan of Zhengzhou University, along with collaborators from Jilin University and Sun Yat-sen University, took a fundamentally different approach [1][2][7].

The team started with highly oriented pyrolytic graphite — a synthetic form of graphite in which carbon layers are neatly stacked and aligned. They placed the material between tungsten carbide anvils and compressed it along the c-axis (pressing directly down on the stacked carbon layers, rather than from the sides) at a pressure of 20 gigapascals — roughly 200,000 times atmospheric pressure — and temperatures ranging from 1,300 to 1,900°C (approximately 2,370 to 3,450°F) [1][2].

The researchers held these extreme conditions for 10 hours, employing what collaborators at Jilin University have termed the "Post-Graphite Phase Gradient Conversion" (PGPC) method [7]. The result: millimeter-sized crystals of hexagonal diamond with purity exceeding 95%.

"These findings resolve the long-standing controversy on the existence of hexagonal diamond as a discrete carbon phase and provide new insight into the graphite-to-diamond phase transition, paving the way for future research," the authors wrote [1][2].

How Hard Is It, Really?

The team subjected their crystals to a battery of tests, including X-ray diffraction, atomic-resolution electron microscopy, ultrasound measurements, and Vickers hardness testing — a standardized indentation method developed by the British firm Vickers Ltd. in the 1920s [2].

The results were striking, if somewhat more modest than the boldest theoretical predictions:

• Vickers hardness: ~114 ± 6.4 GPa along the axial direction; ~106 ± 5.7 GPa along the radial direction [3][5]
• Young's modulus (stiffness): 1,229 ± 15 GPa, slightly higher than cubic diamond on the (100) plane [3]
• Shear modulus: 516 ± 18 GPa [3]

For context, natural cubic diamond typically measures around 110 GPa on the (100) crystallographic plane. The hexagonal diamond's axial hardness of ~114 GPa represents a real but modest improvement — roughly 4% harder, not the 50% advantage that some early models predicted [2][3].

Source: Nature / Lai et al. 2026; various published sources

Data as of Mar 16, 2026CSV

However, the story may not end there. Separate theoretical calculations and some compression experiments suggest that on certain crystal orientations — specifically the (100) plane of hexagonal diamond — the indentation strength could reach as high as 152–165 GPa, a dramatic 50% improvement [5]. Whether those extreme values can be achieved in practice with larger, optimized samples remains an open question.

Oliver Tschauner, a crystallographer at the University of Nevada, Las Vegas, who peer-reviewed the work, called it "the first very accurate characterization of this elusive material," validating what "hundreds of claims" had failed to definitively prove [8].

Beyond Hardness: Oxidation Resistance and Stiffness

The hardness numbers, while headline-grabbing, may not be the most significant finding. The team also demonstrated that hexagonal diamond is more oxidation-resistant than cubic diamond — a property with major practical implications [1][2].

Conventional diamond, despite its legendary hardness, degrades when exposed to high temperatures in the presence of oxygen. This limits its usefulness in many industrial cutting and drilling applications where heat buildup is unavoidable. A material that maintains diamond-like hardness while better resisting thermal oxidation could significantly extend tool life in high-temperature machining environments.

The superior stiffness measurements also confirmed earlier findings from Washington State University's Institute for Shock Physics, which in 2021 produced lonsdaleite crystals large enough to measure stiffness, finding them stiffer than cubic diamond [4].

From Meteorite Curiosity to Industrial Material?

Shan told Interesting Engineering that hexagonal diamond "has potential applications in many fields, for example in cutting tools, in thermal management materials and in quantum sensing" [7].

The potential applications span several high-value sectors:

• Ultra-precision machining and cutting tools: Superior hardness and oxidation resistance could improve tool performance in aerospace and semiconductor manufacturing.
• Thermal management: Diamond is already used as a heat spreader in high-performance electronics; a harder, more thermally stable variant could improve heat dissipation in next-generation chips.
• Quantum sensing: Diamond-based quantum sensors exploit defects in the crystal lattice to detect magnetic fields with extraordinary precision. Hexagonal diamond's different lattice structure could open new sensing modalities.
• Deep-earth drilling: Drill bits that retain hardness under extreme pressure and temperature conditions underground.

These applications sit within a synthetic diamond market that is already booming. Industry analysts project the global synthetic diamond market will grow from $25.5 billion in 2025 to $38.2 billion by 2031, with the Asia-Pacific region commanding over 55% of revenue [9]. The dominant manufacturing methods — High Pressure, High Temperature (HPHT) and Chemical Vapor Deposition (CVD) — already produce industrial diamonds at scale. Adapting these processes to produce hexagonal diamond commercially, however, would require overcoming the extreme pressures involved in the current synthesis method.

The Broader Race for Superhard Materials

The hexagonal diamond breakthrough arrives amid a broader surge in superhard materials research. Computational methods, increasingly powered by machine learning and AI, are accelerating the discovery of novel carbon allotropes with extreme mechanical properties [10].

Recent studies have identified carbon allotropes such as C14-c and C14-p with hardness values around 81 GPa, and a tetragonal carbon allotrope (tp-C64) with a Vickers hardness of 57 GPa — impressive by most standards, but well short of diamond's territory [10]. Hexagonal diamond, by contrast, represents something rarer: a material that doesn't just approach diamond's performance but measurably exceeds it.

Source: GDELT Project

Data as of Mar 16, 2026CSV

Researchers are also exploring boron-carbon-oxygen compounds and other non-carbon superhard materials using informatics-driven design approaches, expanding the toolkit available to engineers who need materials that can withstand extreme conditions.

What Comes Next

The immediate challenge is scalability. The current synthesis produces millimeter-sized crystals under extreme conditions — remarkable for a proof of concept, but far from the centimeter-scale or larger samples that industrial applications would require. The 20-gigapascal pressures and multi-hour processing times also make production expensive compared to conventional synthetic diamond.

Still, the history of synthetic diamond itself offers reason for optimism. When General Electric first synthesized cubic diamond in 1954, the crystals were tiny and the process was impractical for commercial use. Today, lab-grown diamonds are a multi-billion-dollar industry producing gems indistinguishable from natural stones.

For now, the most significant achievement may be conceptual rather than commercial. After six decades of doubt, hexagonal diamond has been proven to exist as a distinct, stable form of carbon — not merely an artifact of defective cubic diamond. That confirmation opens the door to a new chapter in materials science, one in which the hardest substance known to humanity is no longer conventional diamond, but its hexagonal cousin forged in a Chinese laboratory under pressures that mimic the heart of a meteorite impact.