Both the predominant cubic diamond and the less common hexagonal form ("lonsdaleite") exhibit ultrahardness due to the three-dimensional arrangement of C4 tetrahedra with purely sp 3 -hybridization of the carbon atoms. In terms of topology, cubic and hexagonal diamonds are the aristotypes, designated as dia and lon, respectively . This nomenclature is also applicable to other families of carbon allotropes. The introduction of additional carbon atoms results in alterations to the C(sp 3 ) lattice of diamond, giving rise to novel C(sp 3 )/C(sp 2 ) hybrid allotropes that retain the original physical properties, including electronic ones, which can lead to induced metallicity . In the case of the nearest neighbors of carbon, boron and nitrogen, equiatomic cubic boron nitride (cBN) has been synthesized, which is half as hard as diamond but exhibits much higher thermal and chemical stability .