Copper is a reactive metal which will develop a patina over time due to humidity and moisture, an individual's body chemistry, salt air or pollution. This may happen quickly. Copper's reactive qualities are desirable to many people who buy copper as therapy jewelry and who realize copper's anti-microbial properties. The tarnish may transfer to the skin, but is harmless. Copper should not be worn in the shower, while swimming or for sports. To brighten the copper jewelry you may dip it in a mild acid like white vinegar or lemon juice for a minute or two, rinse and buff.
Untreated copper can be oxidized for a darker or antiqued look. You may oxidize it by placing the copper in a sealed jar with one mashed boiled egg and warm water. Keep an eye on it until it reaches your desired results--usually overnight. The sulfur compound in the egg will hasten the oxidation that occurs naturally. You may then buff the high points of the piece to bring back areas of bright copper and create contrast and drama with the darker oxidized areas.