Although the traditional cameo design dates back to the times of antiquity over two thousand years ago, it apparently disappeared during the Middle Ages. The design reappeared during the Renaissance and reached the peak of popularity in modern times during the infamous "Victorian Age". Many of the Victorian era cameos are now commanding top dollar and can be worth thousands while the market is also flooded with a ton of imitation pieces that are reliably worthless and illegally to appreciate in value. Identifying the cheap knock-offs will help you better identify the more valuable pieces that can be lurking at any flea market which will almost certainly be under-priced.
Typically, the best and most valuable pieces are carved while your cheaper imitation pieces will tend to be molded and made from plastic or glass. Carved versions are almost always worth more but some molded madeos made from Bake Lite can actually be more valuable as they are hot collectors' items today. But rather than focusing on the rare exceptions, however, it would probably be more profitable to locate the carved pieces.
One can potentially spot even the best imitation molded pieces because they will have rounded features and a noticeable lack of precise or sharp lines. In addition, the plastic versions will be significantly lighter than a carved piece in the vast majority of cases. Finally, the quality of the other materials used to create the imitations pieces will often reveal clues about the true value of the piece.
Carved cameos made from shell, however, can be difficult to distinguish from the plastic molded versions because they have been purposely designed to perfectly mimic the carved pieces. Cameos made out of shell are often lightweight so you need to be aware that the carved cameos will tend to feature a pair of colors and will be crafted from a single shell.
Carved stone cameos will almost always command the greatest prices because they are hand-crafted and take more time to create. But be careful as the stone portrait may actually be applied or molded. To positively identify, be sure to examine the junction between the plaque and the portrait. The back of the portrait on a carved stone cameo will appear under carved or concave when viewed next to their glass counterparts.
In general, focusing on whether or not a cameo portrait has been carved or molded is the quickest way to identify the most valuable pieces to collect. Both carved and molded pieces have distinctive characteristics that make it easy for the trained eye to identify. By taking the time to learn and identify the traditional characteristics of the carved pieces, one can typically locate under-priced pieces in most flea markets that can be resold for significant profit.