Many zero-phonon lines are known in the absorption spectra of diamonds at 77 °K. Some lines are only produced in irradiated diamonds and others by a further thermal annealing. All these lines split under an applied uniaxial stress and polarization spectra have been obtained for stress applied along the three principal directions [100], [110] and [111]. A parallel theoretical investigation has been made of possible stress spectra for cubic crystals and these have been grouped into seventeen types dependent on the orientational and electronic degeneracy present. The application to the diamond spectra supports the [111] orientation of the centre responsible for the 4152 Å line (N3a) found in many natural diamonds.