If you have ever experienced your jewelry leaving dark marks on your skin, try painting the back or the parts that touch your skin with clear nail polish.
To fix a missing stone, find a nail polish the color of the stone and fill in the spot where the stone is missing. You can also use glitter glue — it comes in a large variety of colors.
To clean costume jewelry, use a soft cloth or soft bristle brush and some dishwashing liquid. For tougher jobs, you can use a plain, basic toothpaste with a soft brush. Just be careful to not scrub too hard and be careful not to scratch any of the stones.
If you lose a jewel and you have it, be sure to use a jewelry glue to repair it. It will ensure your jewels stay put with the right adhesive. You can find it in the glue section at any craft store.