Born of the Stars, Recreated on Earth: Lonsdaleite Is the Hardest Diamond Yet

For decades, scientists have been fascinated by lonsdaleite — a mysterious hexagonal variation of diamond that was once believed to exist only in the aftermath of meteorite impacts. First identified in fragments of the Canyon Diablo meteorite that crashed into Arizona some 50,000 years ago, this rare material has been theorized to be up to 58% harder than conventional diamonds. Now, a team of Chinese scientists has achieved what many considered impossible: the laboratory synthesis of lonsdaleite.

Unlike conventional diamonds, which are built upon a cubic crystal structure, lonsdaleite forms with a hexagonal arrangement of carbon atoms. This subtle shift — often described as a honeycomb-like stacking — makes the material significantly tougher and more resistant to indentation than even the hardest diamond mined from Earth. On the Mohs scale, diamonds rate a perfect 10, leaving no room for a harder category. Yet lonsdaleite challenges that ceiling, suggesting that one of nature’s rarest crystals could redefine the limits of hardness itself.

(Lonsdaleite is named after Dame Kathleen Lonsdale (1903-1971), a pioneering British crystallographer and the first woman elected as a fellow of the Royal Society.)

Until now, natural samples of lonsdaleite found in meteorites were too small, impure, or structurally flawed to provide conclusive proof of its extreme properties. Previous attempts to reproduce lonsdaleite in the lab often resulted in mixtures of graphite, cubic diamond, and other unstable carbon phases. The breakthrough by scientists at the Center for High Pressure Science and Technology Advanced Research in Beijing represents the first time that crystals of significant size and purity have been synthesized. The crystals measured 100 micrometers in width (4/1000th of an inch), about the width of a human hair.

Their process involved subjecting ultrapure graphite to pressures of 200,000 atmospheres and temperatures exceeding 2,500 degrees Fahrenheit. Under these extreme conditions, the graphite layers slid, buckled, and re-bonded into the signature hexagonal lattice of lonsdaleite. By carefully releasing the pressure, the researchers stabilized the new crystals without reverting back to graphite. The results, published in Nature, provide the strongest evidence yet that lonsdaleite exists as a distinct — and harder — form of diamond.

While the discovery is being hailed as a game-changer, the applications won’t be in engagement rings or fine jewelry — at least not anytime soon. Instead, researchers are looking toward high-tech and industrial fields where the need for ultra-hard materials is critical. Precision machinery, wear-resistant coatings, high-performance electronics, quantum technologies and thermal management systems could all benefit from the extraordinary durability and conductivity of hexagonal diamond.

For now, the jewelry world will continue to celebrate the beauty and rarity of conventional cubic diamonds, while scientists begin unlocking the secrets of meteorite hexagonal diamonds, opening the door to an entirely new class of super-hard materials.

Credit: Canyon Diablo meteorite photo by Chip Clark / Smithsonian.