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EP 38
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Harder Than Diamond? The New Hexagonal Diamond Breakthrough

Physics
Materials Science
Engineering
Chemistry
Hosted by Lester Nare and Krishna Choudhary, this episode is a deep dive into one of the strangest and most hard-fought materials science stories in decades: the claim that researchers have finally synthesized bulk hexagonal diamond, also known as lonsdaleite. They break down why this material matters, how it differs from ordinary cubic diamond, why scientists argued about its existence for more than 50 years, and what the new Nature paper actually did to convince skeptical reviewers. The March 4, 2026 Nature paper reports millimetre-sized, phase-pure hexagonal diamond made from highly oriented pyrolytic graphite under high pressure and temperature, and says the material shows slightly higher hardness than cubic diamond. Summary Why hexagonal diamond matters: if real, it is a long-sought carbon phase that could be slightly harder than conventional diamond and useful in extreme industrial settings. The first-principles chemistry behind carbon allotropes, x-ray crystallography, cubic diamond, and the ABAB stacking that makes hexagonal diamond different. How the new team engineered around the “easy path” to ordinary diamond by controlling graphite orientation and pressure direction. Why the peer review mattered so much, and how this new paper intersects with an earlier 2025 Nature paper that also claimed bulk hexagonal diamond.

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Nature·

Synthesis of bulk hexagonal diamond

You know how carbon can be arranged in different ways — like graphite in your pencil or diamonds in jewelry? Scientists have long suspected there's a third arrangement of carbon atoms, shaped like hexagons instead of cubes, that might be even harder than regular diamond. The problem was nobody could make a piece big enough to actually study. This team took ultra-pure graphite crystals, squeezed and heated them under very carefully controlled conditions, and finally grew chunks of this hexagonal diamond big enough to see, hold, and test. Think of it like finally baking a cake you've only ever seen in a recipe book for 60 years — and discovering it tastes almost exactly like the cake you already knew, but slightly better.

hexagonal diamond
carbon allotropes
Nature·

Bulk hexagonal diamond

You probably know that diamonds are made of carbon atoms arranged in a specific pattern—like a perfectly stacked 3D grid. But imagine if those same carbon atoms could be stacked in a slightly different pattern, like a honeycomb, instead of a cube. Scientists have long believed this 'hexagonal diamond' exists because they found hints of it in rocks from meteorite impact sites, suggesting the extreme heat and pressure of a space rock smashing into Earth could create it. But nobody could make it in the lab or prove it was real on its own—until now. These researchers took a special form of super-flat graphite (the stuff in pencils), squeezed it really hard in just the right direction while heating it up, and successfully made millimeter-sized chunks of hexagonal diamond. They confirmed it's real, it's slightly harder than regular diamond, and it holds up to heat really well. Think of it as discovering a new flavor of the hardest material on Earth.

hexagonal diamond
cubic diamond