A novel Faraday-effect element having a refractive index highest along the axis and decreasing toward the surface is produced by immersing a thin elongated glass body containing an oxide selected from the group consisting of paramagnetic- and diamagnetic-type oxides composed of at least first cations, in a bath of salt including second cations having a smaller ration of the electronic polarizability to the third power of the ion radius than said first cations. The bath is maintained at a temperature to permit said second cations to diffuse into said glass body so that said first cations may be substituted by said second cations.