For decades, hexagonal diamond — often called lonsdaleite — sounded almost mythical. It was proposed in 1962 and later reported in meteorites, but many researchers argued that the supposed material was really ordinary cubic diamond full of stacking faults and twins. The central problem was structural ambiguity: defective cubic diamond can imitate the X-ray pattern of a true hexagonal phase, so “proof” kept slipping away. (nature.com)
That changed dramatically in 2025 and again in March 2026, when Chinese research teams reported increasingly convincing syntheses of bulk hexagonal diamond from highly ordered graphite under extreme pressure and heat. In the latest Nature paper, the team produced millimetre-sized, phase-pure samples at about 20 gigapascals and 1,300–1,900 °C, then used X-ray diffraction, electron microscopy and simulations to argue that the material is genuine hexagonal diamond, not just a damaged version of the cubic form. Nature’s news coverage called this the strongest claim yet, and peer reviewer Oliver Tschauner said the telltale diffraction peaks were finally there. (nature.com)
So, does this discovery rewrite the ranking of the world’s hardest materials? Not in any simple way. The new hexagonal diamond was measured at roughly 114 gigapascals in Vickers hardness, only slightly above the roughly 110 gigapascals often cited for natural cubic diamond, although it also seems stiffer and somewhat more resistant to oxidation. That is impressive, but far below older theoretical predictions that pure lonsdaleite might outperform diamond by about 58%. (phys.org)
In fact, “world’s hardest” is already a slippery title. A 2025 Nature Synthesis study reported a nanotwinned form of ordinary diamond with a hardness of 276 gigapascals, and another 2025 Nature Communications paper described diamond wafers exceeding 200 gigapascals. So the real importance of hexagonal diamond may be less about stealing a crown and more about ending a long scientific argument. If these results hold, lonsdaleite is no longer a rumour from meteorites: it is a real, measurable phase of carbon, with possible uses in cutting tools, thermal management and even quantum sensing. (nature.com)
