The assessment of mechanical properties is based on determination of elastic properties by performing finite distortions of the lattice and deriving the elastic constants from the strainstress relationship. The bulk B and shear G moduli may be obtained by averaging the singlecrystal elastic constants using, for instance, Voigt's method based on a uniform strain. In a second step the hardness can be calculated using different models as described in the next subsection. In the present work we consider tetragonal ('glittter' and 'metaglitter') and orthorhombic ('isoglitter') phases. The terms of mechanical stability and elastic moduli can be defined through elastic constants C ij as follow: