A method is presented for the preparation of silica spheres of controllable size with gold nano-particles incorporated as cores. Such cores can serve as labels for tracer studies and provide the dispersion with interesting optical properties. The method comprises the preparation of gold nano-particles in an aqueous host dispersion of small silica (Ludox) spheres, and the growth of the obtained composite particles by the Stober method. This leads finally to the incorporation of the very small gold labels inside about 3% of the silica spheres. The mixture of labeled and unlabeled silica particles can then be transferred to organic solvents after their surface is coated with a long-chain alcohol, which provides steric stabilization. During particle growth, the gold plasmon absorption band is screened by the scattering of the silica layer. The optical properties of the particles in several solvents agree with Mie’s theory for concentric particles. The change in optical contrast between silica and solvent has pronounced effects on the absorption spectrum. The suitability of the tracer particles is demonstrated by dynamic light scattering experiments, which reveal a variation of the measured diffusion coefficient as a function of optical contrast.