Terbium-based Faraday rotators, optical isolators incorporating the Faraday rotators, and methods for forming the Faraday rotators are described. Formation methods include hydrothermal growth methods for forming monolithic single crystals of TbO(OH) as Faraday rotator materials. TbO(OH) can also be used as a starting material in a hydrothermal growth method to form monolithic single crystals of TbxYb(2-x)O3, in which x is between about 0.05 and about 1 or terbium aluminum garnet TAG for use as a Faraday rotator in an optical isolator.