There are some metals, like sterling silver, brass and copper that will eventually tarnish over time. Everything that comes in contact with your jewelry such as sunlight, natural body oils, water, household chemicals, etc. will cause tarnishing. This is the natural nature of metals. The best thing to do is care properly for your bangles and clean them routinely. You can clean the bangles with a jewelry polishing cloth. Also, to slow down the tarnishing process store the bangles in a dry and in closed place. Avoid leaving/having your bangles in humid areas such as, bathrooms, outdoors, and pools. Remove jewelry when showering, cleaning, swimming, and when staying out in sunlight all day.