We report, an optical characterisation by spectroscopic ellipsometry of cobalt (Co) nanocrystals embedded into SiO2 matrix. Co clusters have been formed into a 250 nm thick thermal SiO2 layer on Si substrate during Co+ ion implantation at an energy of 160 keV and at fluences of 1017 ions/cm2 at different temperatures of 77, 295 and 873 K. The obtained nanocrystals have a size range of 2 to 10 nm. Spectroscopic ellipsometry measurements are performed in the 350 to 850 nm spectral range. Experimental data are analysed and interpreted in terms of both nanostructures distribution with depth into the sample and optical properties of the Co nanostructures. We have used a multi-layers model where volume fractions and complex optical constants of nanostructures have been varied. Layers of this model are considered as mixture of Co clusters and silica according to Maxwell- Garnett effective medium theory. Differences between optical constants of bulk and nanostructured cobalt have been observed.