First-principles calculations were performed to investigate the structural feasibility of M and Z phases (novel monoclinic and orthorhombic structures recently reported for carbon) for silicon and germanium. The lattice parameters, bulk modulus, vibrational properties, and elastic constants are calculated using the local density approximation to describe the exchange-correlation energy, while the optical properties are calculated by using Many-Body Perturbation Theory in the G₀W₀ approximation. Our results indicate that silicon and germanium with the proposed crystal symmetries are elastically and vibrationally stable and are small band-gap semiconductors. We discuss the possible synthesis of such materials.