Rhenium diboride

H. Y. Chung, M. B. Weinberger, J. B. Levine, A. Kavner, J. M. Yang, S. H. Tolbert, and R. B. Kaner, Synthesis of Ultra-incompressible Superhard Rhenium Diboride at Ambient Pressure, Jour. Amer. Chem. Soc., 316,436 (2007). 

Diamonds are in fact an exceptional precious stone because of their extreme hardness and light refraction, the second of which makes it sparkle beautifully. A common, but not completely accurate view is that diamonds are the hardest of all known substances. This statement is not easy to prove or disprove as most crystalline substances are anisotropic, which means that their properties (hardness included) depend on the orientation in which they are studied. Ciystals of rhenium diboride (ReB2) are harder in certain orientations than diamonds are in any orientation. The same is true for another substance called ultra-hard fullerene, which—no small irony involved—is another, very rare form of carbon, and can be metastable under any conditions. 

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