It is normal for jewelry to oxidize when exposed to air. To preserve the quality of the jewelry it is important to clean it regularly:
Cleaning: Use warm water with a drop of mild detergent and clean the jewelry carefully with a soft cloth or brush. OBS: Jewelry with freshwater pearls must ONLY be cleaned with water and nothing else.
Daily use: Remove the jewelry before showering, exercising, sunbathing, etc. Take off the jewelry before using body lotion, perfume, hair spray, etc.
Storage: Keep your jewelry clean and dry whenever not used, preferably in a closed box or a polybag. Do not store your jewelry in the bathroom.