Nonlinear materials are of great technological interest for the realization of nanophotonic devices able to control and manipulate light. These properties can be specifically engineered by exploiting the surface plasmon resonance of plasmonic nanocomposites, in proximity of which nonlinear absorption and refraction are largely enhanced. An exploration of the spectral, irradiance, and host material dependence of the nonlinear parameters is important to be able to gain a thorough understanding of the nonlinear response. In the present work, we investigate the nonlinear optical response of and nanoparticles embedded in sapphire by a spectral -scan characterization across the localized surface plasmon band of the samples. We also present a comparison of the results obtained for nanoparticles embedded both in silica and in sapphire, produced under equivalent conditions, to test the influence of the host material. We theoretically describe the spectral trends of both the nonlinear absorption coefficient and nonlinear refractive index by implementing a three-level model, which provides important insights into the main physical parameters controlling the observed spectral features, and the possibility of ab initio composite designs with tailored properties.