The copper collar has coloured this marble post with an appealing verdigris patina. Part of the McKinley Monument at the centre of Buffalo, it is a striking example of coppers tendency to oxidize with moist air, creating copper carbonate (CuCO3). All over Buffalo you can see original copper eavestroughs and rooves, untampered with for over a century, rarely seen in Toronto. Eavestroughs are on my mind as the aluminum set on my house is in sad repair. To have these replaced with copper would be outlandishly expensive, and so remain a pipe dream. If your passion is as big as your pocket book you can purchase copper eavestrough at Spar Marathon. The oxidization process takes about 20 years but if you want to speed things along you can spray the copper with a vinegar, salt and water mixture, resulting in the desired green in 2 weeks or so. For more aggressive techniques consult here.