In the work langasite (LGS, La3Ga5SiO14) samples of different crystallographic orientations and forms were investigated to determine connection between optical heterogeneity and growing conditions. On the plates with two polished sides the interference pictures in bulk (IPB) of crystal were found. The main reason of IPB is heterogeneity of refractive indices. It was established by research of form of the plates’ surfaces and absorption factor in various areas on a plate. The refractive index No and its dispersion were measured by method of a prism in range of wave lengths from 400 up to 800 nm, total measuring inaccuracy was less than 0,0001. It is possible to ascertain that heterogeneity of a refractive index along a direction of growth is much higher than in a perpendicular direction.