Diamond has held the top position on the Mohs hardness scale long enough that the scale itself treats cubic diamond as a ceiling. A study published March 4 in the journal Nature now challenges that ceiling, with Chinese researchers claiming to have produced the first pure samples of hexagonal diamond — a carbon variant theorized since 1962 and debated ever since.
The distinction between the two forms lies in atomic arrangement. Cubic diamond organizes carbon atoms in a cubic lattice; hexagonal diamond, also called lonsdaleite, arranges them in a honeycomb-like hexagonal structure. That structural difference, according to the announcement, produces measurable performance advantages: the new samples are both stiffer and harder than natural diamond, and they resist oxidation significantly better — meaning the material tolerates higher temperatures without surface degradation from oxygen exposure, a property relevant to industrial drilling applications.
A Long-Contested Material
The existence of lonsdaleite has never been settled science. Researchers first detected it in 1967 in ureilite meteorites — fragments originating from the mantles of shattered dwarf planets — including three Canyon Diablo meteorites from Arizona and Goalpara meteorites found in Assam, India. The Canyon Diablo samples showed roughly 30% hexagonal and 70% cubic diamond phases. Skeptics, however, argued that the detections could reflect disordered cubic diamond rather than a genuinely distinct phase, and the debate persisted across decades of follow-up studies.
The core problem was always purity. Previous samples — whether from meteorites or laboratory synthesis — were invariably mixed with cubic diamond, graphite, and other minerals, making it effectively impossible to isolate and measure lonsdaleite’s intrinsic properties. A separate 2025 study had managed to synthesize small quantities in the lab, but the contamination problem remained.
What the New Samples Establish
The Chinese team addressed this directly by producing several pure hexagonal diamond samples each approximately 0.06 inches (1.5 millimeters) in diameter — large enough to conduct rigorous material property measurements. According to the report, structural and spectroscopic analyses supported by large-scale molecular dynamics simulations “unambiguously” confirm the hexagonal diamond phase.
The size threshold matters as much as the purity. Prior synthesis efforts produced quantities too small to test mechanically. At 1.5 millimeters, the new samples cross into a range where hardness, stiffness, and oxidation resistance can be quantified rather than inferred, providing the field with a baseline set of verified physical properties for a material that previously existed more in theory than in practice.
The findings reframe a question that materials science has circled for more than six decades: not whether hexagonal diamond could exist, but whether it could be produced cleanly enough to characterize. The answer, according to this study, is yes — and the properties it exhibits place it above the material long considered the hardest known to exist in nature.
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