Harder than Diamond (from some angles): Rhenium Diboride

“There’s hard, and then there’s superhard. Researchers have designed and made a material capable of scratching diamond — and done it without resorting to harsh, high-pressure methods. The team found that in some circumstances it could actually scratch diamond. “To scratch diamond you have to have something that is at least in the neighbourhood of the hardness of diamond,” says Badding.” Full Article (Nature).