The R-line fluorescence spectroscopy of ruby (Cr:Al2O3) single crystals has been investigated under controlled deviatoric stresses at high confining pressures in a diamond anvil cell. Full stress tensors of the ruby samples were determined based on free-slip boundary condition and cylindrical symmetry. The shift of the R2 line is insensitive to deviatoric stress and represents local mean stress (pressure). When ruby is loaded along the c-axis, the R1-R2 splitting changes linearly with deviatoric stress at a rate of −0.241±0.012 Å/GPa and does not depend on the confining pressure. The splitting increases non-linearly for ruby loaded along the a-axis. The dependencies of the splittings on deviatoric stress are in excellent agreement with theoretical calculations constrained by shock-wave data. The widths of the fluorescence lines increase with growing inhomogeneity in pressure, but are not affected by the deviatoric stresses.