As for 2014, the hardest material on earth is a diamond, a form of pure carbon. It ranks 10 on the Mohs hardness scale and has a rating of between 70 and 150 GPa, or gigapascals on the Vickers hardness scale.
Carbon atoms like to form honeycombed sheets with each corner anchored by a carbon atom. Also, carbon is the only atom that can have four electrons in the second shell that surrounds the carbon nucleus. This allows carbon to create extremely symmetrical and stable crystalline structures. The only natural substance that can break apart the bonds carbon atoms make to form diamonds is another diamond.