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Diamond is considered the hardest material on Earth. Its crystalline structure of carbon atoms in a tetrahedral lattice has a very strong covalent bond, making it incredibly hard to break at room temperature.
But it has always been a difficult task to find something harder than a diamond that can be produced on earth. In the 1950s, scientists discovered a synthetic material called boron nitride (c-BN) that is very similar in structure to diamond, but consists of atoms of boron and nitrogen instead of carbon.
c-BN has been used for decades as a superhard machine tool coating and is also an effective thermal barrier. But despite its strength, c-BN is not able to resist the high temperatures that can melt a diamond at 800 degrees Celsius.
Then, seven years ago, researchers discovered a strange form of boron nitride called wurtzite boron nitride. It has a similar tetrahedral crystal structure to both diamond and cubic boron nitride, but some of the bonds are flexible.
When these bonds are put under pressure they will re-orient themselves by about 90 degrees, a process known as a “bond flip” which increases the resistance to indentation. That’s the reason that two years ago a small quantity of composite containing wurtzite boron made it into the top ten list for indentation resistance.
And now, according to new research, a naturally-occurring rock – lonsdaleite – may be even harder than that. This hexagonal crystalline lattice is 58% stronger than diamond, and it could be used to create tools that are hard as rocks. This rare stone was formed in impact craters by the intense pressure of meteorites crashing to Earth billions of years ago.