A research team led by Sun Jian at Nanjing University has produced recoverable, millimeter-sized crystals of hexagonal diamond, a rare carbon polymorph that prior theoretical work has suggested could be harder than conventional (cubic) diamond. The work, published in Nature on March 5, 2026, reports recoverable samples large enough for direct mechanical testing and detailed crystallographic characterization. The results arrive amid a long-running debate in the literature over whether hexagonal diamond exists as a distinct, long-range-ordered phase, and they could matter for industries that depend on ultra-hard cutting and drilling tools if the findings hold up.
From Meteorite Curiosity to Lab-Grown Crystal
Hexagonal diamond, also called lonsdaleite, was first described as a hexagonal form of carbon in the late 1960s. Scientists identified it in trace amounts inside the Canyon Diablo and Goalpara meteorites, where extreme impact pressures on graphite had converted carbon into a new crystal structure during high-velocity collisions. The grains were tiny, often intergrown with conventional cubic diamond, and far too small for direct mechanical testing. For nearly six decades, lonsdaleite remained a laboratory curiosity defined more by theory and indirect signatures than by experiment.
That theoretical promise, however, was substantial. A 2009 computational study in Physical Review Letters modeled ideal indentation strength under biaxial stress and predicted that lonsdaleite could exceed diamond’s hardness by roughly 58 percent under highly idealized loading conditions. The calculation turned lonsdaleite into one of the most sought-after targets in high-pressure materials science. Yet without bulk samples, no one could confirm or refute the number with physical measurements, and skeptics questioned whether the material existed as a pure phase at all.
Squeezing Graphite Into a New Diamond
Sun Jian’s team tackled the synthesis problem by compressing graphite single crystals inside a multi-anvil press at extreme pressures and temperatures. According to the Nature report, the procedure produced recoverable millimeter-scale crystals of near-pure hexagonal diamond. That size matters: it crosses a practical threshold that finally allows researchers to polish flat facets, mount samples in standard instruments, and perform direct hardness tests rather than relying on nanoscale probes or indirect proxies.
An earlier collaboration involving HPSTAR, Mao, and Yang had also demonstrated bulk conversion of graphite into hexagonal diamond under controlled static conditions. Together, the two efforts suggest the field has turned a corner. Hexagonal diamond is no longer confined to nanocrystalline specks in meteorites or fleeting shock-compression experiments; it can be grown deliberately, recovered to ambient pressure, and handled like a conventional single crystal in the lab.
The new synthesis route starts with carefully oriented graphite and ramps pressure and temperature along a path mapped out by high-pressure phase diagrams. In the multi-anvil press, pressures reach hundreds of thousands of atmospheres, while temperatures climb high enough to overcome kinetic barriers to rearranging carbon’s bonding network. By tuning these parameters, the team steered the transformation away from the usual cubic diamond product and into the hexagonal stacking sequence that defines lonsdaleite.
Hardness Numbers That Challenge Diamond
The most striking hardness claim is reported in a Science Bulletin paper linked to this line of work. Using polished facets cut along specific crystallographic orientations, the researchers pressed a diamond pyramid into the surface and measured the residual impression. On the (100) planes of hexagonal diamond, the Vickers hardness reached 165 ± 4 gigapascals, which the authors describe as roughly 50 percent higher than values for single-crystal cubic diamond under comparable testing conditions. Standard Vickers measurements on high-quality cubic diamond typically fall near 100 to 120 gigapascals, so the reported gap, if confirmed, would be technologically significant.
No independent laboratory has yet published a replication of the 165-gigapascal figure. The number currently rests on a single group’s instrumentation, sample preparation, and analysis protocols. Factors such as surface polishing quality, indenter calibration, and crack formation around the impression can all influence apparent hardness. Until outside teams repeat the experiments on their own hexagonal diamond crystals, the result should be viewed as promising but provisional.
It is also important to keep the 2009 theoretical work in perspective. The predicted 58 percent advantage referred to an ideal shear strength under a specific biaxial loading mode, not to the complex stress field under a Vickers indenter. Real crystals contain defects, grain boundaries, and surface flaws that reduce measured hardness relative to ideal strength. Nonetheless, the fact that both computation and experiment point toward a substantial advantage over cubic diamond strengthens the case that hexagonal diamond occupies the extreme high end of known hardness.
A Decades-Old Skepticism Problem
Part of what makes the new work contentious is a 2014 paper in Nature Communications that argued lonsdaleite does not form a separate phase in most reported samples. That study reinterpreted electron diffraction and X-ray patterns from earlier meteorite specimens as signatures of stacking faults and twinning in ordinary cubic diamond. In this view, what researchers had been calling hexagonal diamond was really just distorted diamond with local variations in layer stacking, not a distinct long-range-ordered crystal structure.
Sun Jian’s group appears to have designed their characterization strategy with that critique in mind. The Nature paper reports synchrotron X-ray diffraction patterns showing sharp reflections consistent with a well-ordered hexagonal lattice, rather than the diffuse streaks expected from random stacking faults. Transmission electron microscopy with selected-area diffraction provided local confirmation of hexagonal symmetry, while electron energy-loss spectroscopy probed the bonding environment to distinguish sp3-bonded hexagonal carbon from possible contaminants or partially transformed regions.
Whether this evidence convinces long-standing skeptics remains to be seen. The 2014 authors set a high bar for ruling out faulted cubic diamond, and they have not yet publicly weighed in on the new bulk crystals. Independent crystallographic studies, especially from groups that previously questioned lonsdaleite’s existence, will be crucial for settling the structural debate. If multiple teams converge on the same hexagonal lattice parameters and defect signatures, the argument that hexagonal diamond is merely damaged cubic diamond will become increasingly difficult to sustain.
Why Harder-Than-Diamond Matters Beyond the Lab
Diamond’s dominance in industrial cutting, drilling, and polishing rests on a simple fact: among commercially available solids, it offers the best combination of hardness, thermal conductivity, and chemical stability. If hexagonal diamond can be produced at scale and its hardness advantage confirmed, the practical consequences could be substantial. Cutting tools for machining heat-resistant aerospace alloys, drill bits for deep geothermal and hydrocarbon wells, and precision grinding wheels for ceramics and composites could all benefit from a material that resists wear significantly longer than cubic diamond.
Even a modest improvement in tool lifetime can translate into major cost savings in sectors where downtime is expensive and tool replacement is frequent. A 50 percent hardness increase, if it translates into comparable gains in wear resistance, might allow thinner cutting edges that maintain sharpness over longer production runs, or drill bits that penetrate abrasive rock formations with fewer trips out of the hole for replacement.
However, the path from millimeter-sized crystals to industrial hardware is far from straightforward. The multi-anvil press technique that yielded bulk hexagonal diamond is energy-intensive and slow, designed for research-scale synthesis rather than mass production. A crystal a few millimeters across is large enough for mechanical tests and small prototype inserts, but far too small for the large segments used in saw blades or the multi-carat compacts found in oil and gas drilling tools. Scaling up would require either vastly larger high-pressure apparatus, more efficient transformation pathways, or entirely new synthesis concepts.
Cost and reproducibility also loom as challenges. Industrial users expect consistent performance from batch to batch; a material that varies in hardness or fracture toughness with subtle shifts in processing conditions will be difficult to qualify. The current studies focus primarily on demonstrating phase purity and peak hardness, not on mapping out process windows or long-term reliability. Bridging that gap will demand close collaboration between academic high-pressure labs and companies that specialize in superhard materials.
A New Platform for Extreme Materials Science
Beyond immediate applications, the ability to grow and recover bulk hexagonal diamond opens a broader scientific opportunity. Because the new method transforms graphite into lonsdaleite under controlled static pressure rather than in the chaotic environment of an impact shock, researchers now have a tunable platform for exploring how defects, impurities, and stress states influence one of the hardest known solids. By varying starting graphite orientation, pressure-temperature paths, and cooling rates, they can systematically probe how microstructure affects hardness, fracture behavior, and thermal transport.
That knowledge could feed back into the design of other ultra-hard materials, from borides and carbides to novel carbon allotropes. It may also refine our understanding of planetary interiors, where carbon-rich phases experience immense pressures over geologic timescales. For now, hexagonal diamond’s status as “harder than diamond” remains contingent on further confirmation. But with bulk crystals finally in hand, the debate is shifting from speculation about what might be possible to careful measurement of what this elusive form of carbon can actually do.
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*This article was researched with the help of AI, with human editors creating the final content.
