A cameo is a miniature carving in relief, usually with contrasting colours for the foreground and background. Originally cut from precious gems, they were first worn as jewellery in ancient Rome. The fashion for them was revived in the 18th century, when hard stones such as onyx and sardonyx - a pink and white agate -- were employed as materials. These cameos are now exceedingly rare, and from the mid 19th century onwards most cameos have been carved from conch shells, a much easier medium to work with. In recent times, fake resin cameos have also appeared on the market.
Examine your cameo carefully, as you'll need to know a little bit about it before you can price it. Let's check first to see whether, as is likely, it is made from a conch shell. To do this, simply look at the back. If the back is concave, then you're the proud owner of a "shell cameo." If the back is flat, with striations, then the cameo is made of resin and sadly worthless.
Look at the front of the cameo and inspect the design. Most cameos made from 1900 onwards are adorned with the head and shoulders of a young lady. If the lady is full figure, or the design is of something completely different, then this is a strong indicator of an earlier date and a more desirable piece.
Assess the quality of the carving by looking at fine details such as hair and drapery, lips and eyebrows. Some cameos show enormous skill, others can be quite crude, and prices vary accordingly.
Check the mount for gold or silver marks and a smooth, professional finish. The mount should be silver at the very least, and the best cameos would have been set into gold. A base metal setting is a sign of a cheap modern cameo that will command little or no interest. (For more help with gold and silver marks, see Resources.)
Browse cameos on Internet auction sites, using the observations you've gleaned from the previous steps to help you come up with search terms for the site's search engine. For instance, if you've established that your cameo is made out of shell, has an 18 carat gold mount and depicts a full figure rather than just a bust, then type combinations of those terms into the search engine. Scan the results until you spot a couple of items that look like yours, then use the site's tracking tool to follow them through to their conclusion. Collecting the final "hammer prices" on a number of similar items will give you a good sense of what your own item is worth on the open market at that time. (See Reference 2.)
Search the online catalogues of traditional auction houses for more results. Many auction houses now publish the catalogues of both their current and previous sales online. The current catalogue carries the auctioneer's estimate of the price an item is likely to achieve, while the past catalogues will have final hammer prices included. These sites are a good source of prices for more expensive, high end cameos, but cheaper, lower quality ones are usually placed in job lots in traditional auctions. (For more help in finding auction house sites, see Resources.)