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. Always look for clean crisp carvings. A damaged cameo cannot be fixed.
Some themes are very rare and therefore worth more because of the scarcity.
Cameos are alive and they need to breathe. They do not like heat. They should be oiled at least once a year. I use mineral oil. I do not like olive oil because if it is not completely removed it will turn a cameo yellow and smelly.