Diamond, natural gem and man-made , is recognized as the hardest material having wide applications as in cutting tools and abrasives. However, its relatively low thermal and oxidation stability was a major drawback in operating conditions that started the era of ab initio search for other compounds of light elements with similar mechanical properties, within the B-C-N system, such as dense boron nitride BN , low-compressibility carbon nitrides C 3 N 4 and ternary heterodiamond BC 2 N . Note that all these compounds are isoelectronic with diamond expressed as C 2 in face centered cubic system, i.e. with 8 valence electrons (C: 2s 2 , 2p 2 ). BN has 8 valence electrons (B: 2s 2 , 2p 1 and N: 2s 2 , 2p 3 ), C 3 N 4 has 32 electrons (48), and BC 2 N has 16 electrons (28).