Diamond is a cubic crystalline form of carbon. In addition to the diamond, following forms of carbon are known:
Diamond is metastable at room temperature and normal pressure. However, the activation energy for the phase transition in the stable modification (graphite) is so high that a conversion to graphite at room temperature practically non-existent.
The diamond lattice is anisotropic, ie, from different viewing angles resulting from the spatial arrangement of atoms and their bonds have different properties.