The nature of langasite crystal´s coloration

La₃Ga₅SiO₁₄ (LGSO) crystals grown from stoichiometric starting material are characterized by the absorption band edge in the UV-region at wavelengths beginning with λ=0.29 μm, the existence of absorption bands with λ_max=0.36, 2.9, 3.75 μm, as well as by ρ>10 ¹² Ohm*cm, ε_y<25, ε_z<58 and tgδ<0.002. The presence of 10^-3-10^-2 mass% of rhodium in the crystals lead to the appearance of an absorption band with λ_max=0.58 μm. Absorption at the said wavelength increases while heating the samples at T>623 K in electric field with E>400 V/cm. In this case ρ decreases by 100-1000 times, tgδ increased by an order. The band at λ=0.58 μm disappears if the colored samples are annealed in hydrogen at 973 K and is restored by heating the colorless samples in oxygen or air above 1173 K; the intensity of the absorption band of the OH-group (λ=2.9 μm) changes. The variation of crystal composition (Ga or Si), as well as the introduction of Al or Ti ions in LGSO do not result in the emergence of absorption in the red region. However, in this case the character of absorption in the UV-region noticeably changes. Recommendations for obtaining transparent crystals are given