The main purpose of metallic deposits is reflection.
The most common metals are Aluminum, Gold, Silver but we also make mirrors in Nickel, Rhodium, Iridium .... The mirrors are protected or not. In the visible range, the protection is generally Silica, SiO2 and protection of gold deposits with ZnS.
The reflective deposits can also be made of dielectrics for improved reflectivity (OPTAL S).