Brass and silver oxidize naturally with time and with contact to the skin. This is a beautiful and natural look of the metals, but it's possible with a bit of cleaning once in a while to maintain the new, shining, bright look of the metal. Use a bit of lemon or vinegar to soak the jewel, and a soft cloth to rub the jewel until it shines again. There are also special cloths (that contains a sort of gentle acid) for cleaning metals, usually sold in jewelry stores.
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