The natural darkening of aged silver is referred to as patina, and is considered very desirable because it shows the detail of the workmanship in jewelry. In charms, for example, the low or engraved areas take on a darker patina and the surfaces are a bit more lustrous. Most collectors of vintage silver jewelry prefer the patina that develops with age and wear, rather than the bright shine usually seen on new silver pieces. Wearing your jewelry constantly while working, relaxing and bathing creates hundreds of tiny scratches on the surface that enhance the natural soft sheen of the silver, leaving the recessed areas dark.
When cleaning silver, care should be taken not to damage the piece or take away the beautiful patina created from wear. There are mild jewelry solutions available in stores and at jewelers that will soak off the dirt without damaging the patina. A very soft toothbrush can be used to get into crevices. The silver should then be rinsed thoroughly & blown dry to to avoid spotting. One issue to be aware of when cleaning silver jewelry with a liquid solution is to make sure there aren't any parts that will be destroyed if they get wet. For example if a Stanhope charm, which has a tiny viewing lens with a picture inside, becomes wet the picture will be damaged the charm will be ruined.
Another effective cleaning method are soft cloths that are treated with a special chemical agent and non-scratching micro-abrasives. To use one of these cloths first clean any dust or surface dirt from the silver piece and then very gently rub the surface to be polished. These pre-treated cloths are available on the internet and jewelry stores under a variety of brand names. They're effective and easy to use.
Harsh dipping solutions should not be used under any circumstances, as they will damage the surface of fine old pieces. If silver polish is used, make sure it's a gentle paste polish. Wipe the polish on with a soft cloth, and try not to get the paste into grooves or crevices. You want to avoid removing the darker color from the low areas in order to retain the beautiful contrast and relief.
The best solution for cleaning silver jewelry is to wear it continuously, including while doing dishes, bathing & swimming. This will make your sterling rich-looking and lustrous, and give you the most enjoyment of your treasures.