They are apprentice carved and mass produced. Most are very poorly carved. The most important advice I can give you is to buy from a reputable dealer who knows about cameos. I have found few do. You need to decide the right size for you.
Most people start with a head and go on from there.
Older cameos are set in brass, pinchbeck, gold filled, silver vermeil, or silver. A cameo not in its original setting takes away some of the value. There are many fine unset cameos however that were never set. Many tourists brought home unset cameos from Italy. Many cameos were carved in Italy and set in France, England and Germany.