Until very recently diamond was the only material qualified as ultrahard. Despite its unique properties i.e. extreme hardness, elevated thermal conductivity, large band gap, high electron and hole mobility making it suitable for diverse basic research and applications, the reactivity of diamond with oxygen and ferrous metals constitute a drawback. Furthermore, diamond's hardness drastically drops with temperature. As consequence, there is a growing demand for other advanced materials that stimulated the search for novel ultra-hard thermally and chemically stable phases (cf. and references therein).