We have studied the effect of the photonic band gap on the visible luminescence of opal photonic crystals. The results indicate that the position of the photonic band gap in the luminescence and reflection spectra of opal depends on the photon propagation direction. Optical measurements have been used to determine the refractive index of opal and the diameter of silica spheres. We have assessed the effect of oxygen annealing on the intrinsic luminescence spectra and crystal structure of opal. The emission centers in synthetic opal have been tentatively identified. The visible luminescence intensity in opal is shown to be anisotropic, in accordance with the frequency and angular dispersions of the first photonic band gap in opal.