For Gold you may look at

https://refractiveindex.info/?shelf=main&book=Au&page=Johnson

and for Co3O4

Article Precipitation method and characterization of cobalt oxide na...

Best Regards

The following link may be useful for you https://www.researchgate.net/publication/222015934_Optical_electrical_and_magnetic_properties_of_Co3O4_nanocrystallites_obtained_by_thermal_decomposition_of_sol-gel_derived_oxalates

The refractive index will depend on the frequency or wavelength you require. For gold the following are references for infrared values

M. A. Ordal, R. J. Bell, R. W. Alexander, L. L. Long, and M. R. Querry, “Optical properties of fourteen metals in the infrared and far infrared: Al, Co, Cu, Au, Fe, Pb, Mo, Ni, Pd, Pt, Ag, Ti, V, and W,” Appl. Opt. 24(24), 4493–4499 (1985).

P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972).

R. L. Olmon, B. Slovick, T. W. Johnson, D. Shelton, S.-H. Oh, G. D. Boreman, and M. B. Raschke, “Optical dielectric function of gold,” Phys. Rev. B 86(23), 235147 (2012).

If you have the electrical conductivity then you can use the Drude theory to calculate the refractive indices (real and imaginary) at any frequency / wavelength - see e.g. K. Gallacher et al., "Design and simulation of losses in Ge/SiGe terahertz quantum cascade laser waveguides" Optics Express 28, 4786 (2020) - this can be downloaded at http://www.opticsexpress.org/abstract.cfm?URI=oe-28-4-4786

I hope this helps.

Douglas

https://www.sciencedirect.com/science/article/abs/pii/S0921510709002773?via%3Dihub