Costume jewelry may contain a variety of metals, including iron, which is prone to rust -- especially in the case of rings because they're likely to come in contact with moisture or sweat that oxidizes the iron. Drying off the metal ring after you wash your hands helps minimize rusting, but if rust does develop, take these steps to help make your jewelry look new again.
Mix two parts baking soda or borax with one part lemon juice to form a paste, combining ingredients until it's as thick as toothpaste. Add baking soda if the paste is runny, or more lemon juice if it's too thick.
Smear the paste onto the ring, making sure to thickly coat the rusty areas.
Allow the paste to dry completely, then rinse under water.
Cut a potato in half and dip one half into salt or baking soda. Scrub away any remaining rust.