In this article we present the experimental results for the pressure variation of the refractive index of sapphire up to 16 GPa, obtained with an interferometric method, using the diamond anvil cell. In the range of hydrostatic pressures, up to about 12 GPa, the analysis of the results with the classic Lorentz–Lorenz approach provides a nearly linear relation between polarizability (&agr;) and volume, corresponding to a constant strain polarizability parameter. For pressures above 12 GPa, there is a substantial nonlinear deviation, associated with nonhydrostatic effects inside the diamond anvil cell.