Obviously the most important elements in a Faraday Rotator and Optical Isolator are the Faraday Rotator Rods which made of Magneto Optic Glass 

The characteristics that one looks for in a Faraday Rotator Optics, include a high Verdet constant,low absorption coefficient,low non-linear refractive index and high damage threshold. Also,to prevent self-focusing and other thermal related effects,the optics should be as short as possible.The two most commonly used materials for the wavelength 400-1600 nm range are terbium doped borosillicate glass (Tb glass) and terbium gallium garnet crystal (TGG),but TGG crystal can't grow large size and too expensive. In general,most of Faraday Rotator Elements are produced by type MR3-2 glass considering the cost

Faraday Rotator Glass Specification
Type MR3-2,Verdet constant -0.33min/Oe*cm at 632.8nm
Type MR4,Verdet constant -0.38min/Oe*cm at 632.8nm

* Maximum inclusion size < 0.040 mm

* Maximum total inclusion cross section < 0.04 mm*2 per 100 cm*3

* Bubbles,inclusions,cord,striae: Optical grade

* Bulk laser damage threshold > 25 J/s.q.cm (20nsec pulse at 1064 nm)

* Strain birefringence extinction ratio to perpendicular polarization ≥ 40 dB (by special annealing)