Abstract <p>The effect of impurity cations on the optical and electrophysical properties of La₃Ga₅SiO₁₄ crystals (sp. gr. <i>P</i>321), inactive and activated by Cr and Mn ions (1000 ppm), is shown. The optical properties of these crystals indicate different mechanisms of point defect formation, which leads to difference in their electrophysical properties. For the La₃Ga₅SiO₁₄&#xa0;:&#xa0;Cr crystal, the dc electrical conductivity in the directions parallel (σ_||<i>c</i>) and perpendicular (σ_⊥<i>c</i>) to the optical axis in the temperature range of 673–825 K and the static permittivity (ε_||<i>c</i> and ε_⊥<i>c</i>) at 297 K were measured by impedance spectroscopy. The data obtained are compared with the results of previously published measurements for La₃Ga₅SiO₁₄&#xa0;:&#xa0;Mn.</p>