Faraday effect crystal Terbium Gallium Garnet (Tb3Ga5O12 or TGG) is a crystal material for optical isolator devices. Optical isolator devices make use of the non-reciprocal Faraday effect in Terbium Gallium Garnet (Tb3Ga5O12 or TGG) . Faraday effect is the rotation of the plane of polarization of the light beam as it is transmitted through TGG crystal in the presence of an external magnetic field coaxial with the light. The polarization rotation is in the same sense regardless of the direction of the propagation of the light. An optical isolator is a Faraday rotator combined with suitably aligned polarizers which allows light to pass in one direction only. Terbium Gallium Garnet (Tb3Ga5O12 or TGG) has twice the Verdet constant of a Terbium-doped Glass.