Potassium terbium fluoride is a recently developed magneto-optic material which has been proposed for use as an optical isolator. We have performed measurements of the refractive index, thermo-optic coefficient, and stress-optic coefficient of this material. We present a temperature dependent Sellmeier equation along with calculations of temperature and refractive index profiles at various pump power levels in a diode pumped laser. The data are critical to the design of laser systems in which optical isolators are employed.