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Diamonds are well-known for their unbreakability, but they don’t have the monopoly on being hard. Every few years scientists herald the discovery of a new material that’s harder than diamond. But how do they test this claim?

Materials are measured for their strength by the amount of force needed to cause a dent in them. This is called the Vickers hardness test and it involves using a diamond-tipped instrument. The force used and the size of the resulting indent are then used to calculate the material’s hardness. Mild steel scores around 9 GPa and diamond has a value of 70 – 100 GPa.

But there are claims that two natural materials could be even harder than this. Hexagonal carbon, also known as lonsdaleite, and cubic boron nitride both have the potential to be harder than diamond, but the problem is they are too metastable – meaning they need a lot of heat and pressure to transform into their harder forms.

Zicheng Pan at Shanghai Jiao Tong University in China and colleagues have now simulated how atoms in these materials behave when subjected to stress. They found that wurtzite boron nitride could be made to be 18 per cent stronger than diamond, and lonsdaleite 58% harder.

Although the researchers haven’t yet tested their claims with physical experiments, their work is an important step in understanding how these superhard materials might be created in the lab. That would allow us to design them to order and create materials that are far tougher than anything found naturally.