TL;DR
Costume jewelry fails in two ways: the plating wears through to the base metal, and the glue holding stones in gives out. Both are made worse by water. Clean it with a barely damp cloth and dry it immediately, never soak it, never use an ultrasonic cleaner, and store it dry and separate. A clear sealant on the inside of a ring buys real time.
What costume jewelry is made of
Most fashion jewelry is a base metal -- brass, copper, zinc alloy, or steel -- with a very thin decorative layer on top. That layer might be gold plating, silver plating, rhodium, or a lacquer.
Plating on inexpensive pieces is measured in millionths of an inch. It is thin enough that ordinary polishing removes it, which is why every technique that works on solid gold or sterling is wrong here.
Stones are usually glass, acrylic, or crystal, and they are usually held in with adhesive rather than metal prongs. Adhesive softens in water and fails in hot water.
Pearls on costume pieces are typically glass or plastic beads with a coating that flakes when soaked or rubbed with anything abrasive.
That combination -- thin coating, water-soluble glue, coated fake stones -- is why the whole category needs the lightest touch of anything in a jewelry box.
How do you clean costume jewelry?
Wipe it with a soft cloth barely dampened with water, or with a tiny amount of mild soap, working around stones rather than over them. Dry it immediately and completely with a second soft cloth, then let it air out before storing. Do not soak, rinse under a tap, or use an ultrasonic cleaner.
The safe method, step by step
1. Dust it dry first. A soft dry cloth or a clean makeup brush lifts loose dust out of settings without introducing any moisture at all. Do this every time before anything wet.
2. Dampen a cloth, not the jewelry. Barely damp. Wring it out until it feels cool rather than wet.
3. Wipe the metal surfaces. Light pressure, straight strokes. No polishing compounds, no treated silver cloths, no abrasives.
4. Work around stones, not across them. Use a slightly damp cotton swab along the edges where grime collects, keeping water away from the base of each stone.
5. Dry immediately with a clean dry cloth. Then leave the piece in open air for an hour before it goes in a box. Moisture trapped against base metal is what starts corrosion.
6. For crevices, use a dry soft brush. A clean dry toothbrush gets into filigree without water.
If a piece is very greasy, one drop of mild dish soap on the damp cloth is acceptable. Never let a piece sit in soapy water.
Why does costume jewelry turn skin green?
Green skin comes from copper in the base metal reacting with sweat, skin acids, lotion, and perfume, forming copper salts that transfer to skin. It is harmless and it means the plating has worn through somewhere. A thin coat of clear jewelry sealant or clear nail polish on the inside of a ring or the back of a pendant blocks the contact.
Making plating last longer
Plating is a consumable. You cannot restore it at home, but you can slow the loss.
Put jewelry on last. After lotion, perfume, hairspray, and makeup. These are the main things that eat plating.
Take it off before water. Showers, dishes, swimming, hand washing. Water finds its way under plating at any nick and lifts it from underneath.
Take it off before sweating. Sweat is salty and acidic, and it is the fastest route to a green mark and a worn spot.
Wipe it after wearing. Ten seconds with a dry soft cloth removes the skin oil that would otherwise sit on the surface all week.
Seal contact points. Clear nail polish or a clear jewelry sealant on the inside of a ring band, the back of a pendant, or the post of an earring prevents both green skin and further plating loss. Reapply every couple of months as it wears.
Solid metal pieces are much more forgiving and can be washed properly, as covered in how to clean gold jewelry at home safely.
What never to use
Ultrasonic cleaners. Vibration loosens glued stones and drives water under plating. This is the single fastest way to destroy a costume piece.
Silver dips and tarnish removers. They strip plating in seconds. If a plated piece has darkened, that is usually the plating wearing thin, not tarnish that can be removed.
Silver polishing cloths and polish pastes. They are mildly abrasive by design, which is fine on solid sterling and fatal on plating. The methods in how to clean tarnished silver jewelry do not transfer here.
Toothpaste and baking soda. Abrasive.
Rubbing alcohol and acetone. They dissolve adhesives and can craze acrylic stones and lacquer finishes.
Bleach and household cleaners. Corrosive to base metals, and never to be mixed with anything else in any case.
Hot water. It softens the glue holding stones in place.
Storing it so it stops degrading in the box
Costume jewelry deteriorates in storage even when nobody touches it.
Keep it dry. A bathroom drawer is the worst place in the house, because humidity plus base metal equals corrosion. A bedroom drawer or a closed box in a dry room is much better, and a silica gel packet inside it controls humidity for free.
Store pieces separately in small pouches or compartments so nothing rubs against anything else. Rubbing removes plating faster than wearing does.
Keep it out of direct sunlight, which fades enamel and yellows some plastics and adhesives, and do not bundle pieces with rubber bands, which release sulfur compounds that mark metal.
Source: Federal Trade Commission Jewelry Guides on plating and metal disclosure
When a piece is done
Some damage is not fixable at home or economically at a jeweler.
Plating worn through to a different-colored metal underneath cannot be spot repaired. Replating a costume piece usually costs more than the piece did.
Missing stones can be reglued with a jewelry-specific adhesive, and that is worth doing on something you like. Match the adhesive to the stone material and use a very small amount.
Cloudy or flaking coating on faux pearls is permanent. Discolored enamel that has chipped is permanent.
Corroded base metal that has gone rough and dark under the plating means the piece is at the end of its life. Accessories with similar plated hardware, like the ones in how to clean hair clips and claw clips, reach that point the same way, and the general triage is covered in cleaning the personal items you use every day.
FAQ
Can you clean costume jewelry with soap and water?
Only very sparingly. Use a cloth barely dampened with water and a single drop of mild soap, wipe, then dry immediately. Never soak a piece or rinse it under a tap, because water lifts thin plating from underneath and softens the adhesive holding stones in place.
Why does costume jewelry turn my finger green?
Copper in the base metal reacts with sweat, skin acids, lotion, and perfume, forming copper salts that rub off on skin. It is harmless, and it means the plating has worn through. A thin coat of clear jewelry sealant or clear nail polish on the inside of the band blocks the contact.
Can you use silver polish on plated jewelry?
No. Silver polishes and treated polishing cloths work by removing a microscopic layer of metal, and plating is only a few millionths of an inch thick. One session can take it down to the base metal. Wipe plated pieces with a dry or barely damp soft cloth instead.
Can costume jewelry go in an ultrasonic cleaner?
No. Ultrasonic vibration loosens the adhesive that holds rhinestones and faux pearls in place and drives water underneath the plating. It is the fastest way to ruin a costume piece. Ultrasonic cleaning is for solid metal with hard, securely set stones only.
How do you keep costume jewelry from tarnishing?
Wipe it dry after each wearing, store each piece separately in a dry place away from the bathroom, and add a silica gel packet to the box. Put jewelry on after lotion and perfume, and take it off before showering, swimming, dishes, or exercise.
Can you fix a rhinestone that fell out?
Often yes. Clean both surfaces gently, let them dry completely, and use a small amount of a jewelry-specific adhesive suited to the stone material. Use very little, since excess glue that squeezes out around a stone dulls its edges and is difficult to remove once cured.
Small things stay nice in a home that is not dusty. If keeping up with that is more than you want to handle, get a quote or read more about house cleaning services.