Rust and Soot
Marble lends itself to decor in metal, and copper, silver and steel objects really complement its lines and smoothness. Some metals, however, also stain the polished surfaces and allow for oxidation and rust buildup. Early on, rust is easy to remove with a wire brush that can reach into crevices without being firm enough to damage polished surfaces. Set-in stains will need a poultice or other powder and chemical treatment, sometimes with a rust remover, to draw them out.
Black soot and fireplace buildup are similar in their dark, penetrating effects, but they can usually be removed with just a soap and water cleaning or gentle baking soda scrub followed with cloth rinsing or wet paste treatments set in overnight then rinsed and repeated.
Both of these stains may require some scraping to get at deeper discoloration, which would lead to some rebuffing or re-polishing too.