First of all, you need to think what sort of jewellery you have, as different things are cleaned in different ways. Is your piece of jewellery set with harder stones (not just the obvious diamonds, sapphires and rubies, but also cubic zirconia, amethyst, topaz and rhinestones), softer stones (and stone-like substances) such as turquoise, opal, lapis lazuli, coral and amber, or is it a string of pearls? Each of these needs to be cleaned in a different way.
Jewellery involving the harder stones (or no stones) is the easiest to clean. Treat them like you would your teeth: in other words, scrub them with toothpaste using a soft toothbrush. The soft toothbrush prevents any scratching. Diamond, being the hardest of all substances, is unlikely to be scratched by some puny piece of plastic. Gold, however, is pretty soft. A toothbrush can also reach into all the little nooks and crannies - chains, claws, filigree... Toothpaste rinses off in plain water very easily, leaving no residue behind, as well as being an excellent cleaner. Some people like to use an ammonia solution or dishwashing liquid, but toothpaste doesn't have the fumes and is gentler on your skin. The only real precaution you should take when cleaning this type of jewellery is to remember to do it in a bowl rather than in the sink, where the plug lurks like a black hole ready to swallow your precious engagement ring.
Dry your jewellery with a soft cloth. Don't use a paper towel unless you fancy picking bits of paper out of a claw setting.
Jewellery involving the softer stones and stone like substances should not be cleaned with ammonia or soap, and probably shouldn't be cleaned with toothpaste, either. This is because they absorb water and other substances very easily. In case you're not sure which the softer jewels are, here's a quick list: amber, coral, lapis lazuli, malachite, opal (my favourite), turquoise and organic substances like bone and shell. These should be cleaned with a soft damp cloth with just water. The same applies to anything that involves glue. If you have a ring set with any of these gems, you should also remove the rings before doing anything wet or messy. Put the rings in a special dish near the sink - in fact, that's good advice for any ring. Pearls should be cleaned as little as possible. In fact, wearing pearls seems to make them shinier rather than duller - maybe living, breathing human skin works like chamois leather on them. If your pearls really are grubby (or if they've got sweat on them), then use a mild saline (salt) solution. Never spray perfume on pearls or use ammonia - this will stain the pearls.
If in doubt about any piece of jewellery, then ask your jeweller about the best way to clean and care for it. Even if you didn't buy your gems off them (e.g. if you inherited Great Aunt Margaret's emerald earrings), ask anyway - they'll probably be happy to establish a relationship with you, as you may one day need to re-set the jewels - or even buy more.
