Harder Than Diamond?

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Diamonds may still be a girl’s best friend, but they’re about to lose their title as the world’s hardest natural substance. Scientists have discovered a mineral called lonsdaleite that is 58% harder.

Lonsdaleite is made of carbon atoms that are arranged differently to those in diamonds. It is produced in meteorites that crash into Earth and can also be synthesized in a lab. It is traditionally rated between 7 and 8 on the Mohs hardness scale, which measures how much force it takes to make an indentation into a material’s surface. For comparison, a diamond is a 10. The hardness of materials can be measured by using an instrument that pushes down on the surface and the resulting indentation size and depth is recorded. This is known as the Vickers hardness test and is typically carried out using a diamond tip with a square base.

A few years ago, scientists thought they’d found a substance harder than diamonds. It was a form of carbon, but it had an unusual structure that allowed it to withstand extraordinary levels of stress. However, it turns out that this material has its limits too. The tiny flaws in the crystals that make it so strong can be stretched to breaking point by very high pressures.

Researchers at the University of Nevada and Shanghai Jiao Tong University have been studying another kind of carbon with potential for superhardness. They simulated how the atoms in the rare, hexagonal form of boron nitride (wurtzite BN or w-BN) would react to stress. Their calculations revealed that w-BN could be compressed into a crystalline lattice that was 18% more resistant to indentation than diamonds.