The pressure dependence of the refractive index of tholeiite basalt glass was studied in the range 0-5.0 GPa with an interference-polarization microscope and a diamond anvil cell. The non-linear change of the refractive index of basalt glass, with a kink around 2.0-2.5 GPa, is similar to the behaviour of the refractive index, density and elastic properties of silica glass under pressure. The comparison of data for glasses with continuous silicon-oxygen frameworks shows that the strain-polarizability potential Λ (photoelasticity theory), describing the change of refraction R with increasing density ρ: (Λ=-∆R ρ/R0∆ρ) is nearly constant. Using the value Λ=0.21 and the refractive index data, the compressibility of the investigated basalt glass under pressure up to 5.0 GPa was calculated by the equation: ∆ρ/ρ=6n∆n/(n2-1)(n2+2)(1-Λ). The results of the calculation agree with the compressibility of tholeiite basalt glass determined by measurement of the sizes of glass samples under pressure using photographic equipment. The compressibility of basalt glass is higher than that for plagioclases, resulting in the possibility of relative density inversion of deep-seated basalt melts and plagioclase crystals. These data reinforce the model of anorthositec crust generation on the Earth and on the Moon by inverse plagioclase crystal fractionation at high degrees of melting of basic rocks.