The optical properties and unit cell dimensions of cordierite were studied for the purpose of presenting methods for the estimation of the (Fe⁺²+Mn):(Mg+Fe⁺²+Mn) ratio and the degree of distortion (deviation from the hexagonal indialite structure) of the minerals. The results are summarized in determination diagrams given in this paper. The unit cell dimensions of indialite were compared with those of cordierites and the relation was established that the unit cell volume of cordierite increases with the increase in the degree of distortion. The study also revealed that the optical properties of volcanic cordierite differ considerably from those of high cordierites inverted from low subdistortional cordierites by heating and the cause of this difference is discussed.