A cured coating for a siliceous material, preferably an optical fiber, comprises a thermally cured polyorganosilsesquioxane having an oxide powder dispersed therein. The oxide powder has a refractive index from about 1.2 to about 2.7 and includes particles having a particle size less than about 100 nanometers. The cured coating has adhesion to the siliceous material and is transparent to ultraviolet radiation.