I use professional-quality polymer clay and various metals for my designs.
Polymer clay, when it is good quality and properly cured, is a very sturdy material and should last a very long time. You may notice slight flex in your earrings, depending on their thickness. This is normal and means they were properly made. (Please do not bend them intentionally, though!)
If you need to clean dirt or smudges off the clay, gently use some rubbing alcohol on a cotton swab to wipe soiled areas. Be very careful not to damage areas with surface techniques such as painting and gold leaf.
Flat earring posts are all stainless steel. Hooks and ball studs are usually sterling silver plated or 18k gold plated. If you have any allergy concerns about the design you're interested in
Many of the gold-toned earrings and accents I use are made of raw brass. Over time, if you notice any dulling or discoloring of the brass, you can easily restore its shine with some brass polish and a soft cloth. Gold and silver elements should not require any polishing.