Since costume jewellery is not made of the same high-quality materials used to make fine jewellery, it is less resistant to wear and damage. Though the monetary value of a costume piece may not be high, it may possess a great deal of personal value. One advantage to costume jewellery is that if it breaks or tarnishes, it is entirely possible that you can fix it yourself with supplies from your local craft store.
Clean the plating with plating solution or polish. The most common damage to jewellery plating is tarnish or lost lustre. Find a chemical product formulated for that exact purpose. Buff the plating with polish to restore shine and even tonality.
Glue a broken piece back into place with jewellery glue or epoxy. It is easy for a piece of plating to fall off or even break off under the wrong circumstances. Use an adhesive formulated for jewellery or metal to ensure that the pieces adhere well. Metal is not the easiest thing to glue together, so find something designed with this task in mind. Apply pressure to the two pieces as they dry, to facilitate a lasting bond.
Replace missing or broken plating with another material if necessary. If you do not have the missing piece of plating and cannot find a piece of plating suitable for the repair, remedy the loss with another material. Fill holes or gaps wit jewellery resin or clay which you can paint once it hardens. Replace a missing link with a link made of another material which you can also paint to match the rest of the article.
Paint a new component or a bare spot with metallic gold paint. There are two kinds of metallic paint. The first, used in wood, paper and cloth crafts, has a metallic sheen in the paint. The second kind is formulated to look like metal once it dries. Confirm that you have the second kind. You may find it with jewellery making supplies, but there are also spray paints and metalworking paints that can accomplish this goal.
Add a new piece to the article of jewellery. Cover-up a bare spot with a rhinestone, bead, button or some other element which will fit in with the piece and cover the damaged area. Affix the new component with jewellery glue or epoxy.
Repurpose the jewellery, if all else fails. If there is no way to salvage the piece as a whole, take the part that is still wearable and turn it into another piece altogether. Missing plating may ruin the piece as-is, but does not have to be a total loss.