I am no expert, I googled this and it says to put a submersible piece of copper in vinegar mixed with baking soda. Another one says to make a solution of vinegar, salt, baking soda and flour. There were ones that said make a paste. I would try vinegar and soda first.
Get a bucket fill it with enough vinegar to submerse your rock and add some baking soda. Put your copper in and let it soak for fifteen minutes. If that doesn't work. Type cleaning oxidized copper into your browser and try the next method.