To find the value of antique cameo jewelry, first determine if it is made of conch shell or resin. If it is made of conch, it is valuable, so check the design on the front to determine if it was made before or after the 1900s. Check the mount's metal for gold or silver marks. Research the value of a comparable cameo on an online auction site or a collector's catalogue, or hire an appraiser.
A cameo is made from a conch shell if the back of the image is concave. A resin cameo has a flat back, and is not very valuable. A cameo was made before 1900 if the lady is represented in full figure, or the image is not a woman at all. It is made after 1900 if the image is the silhouette of a woman's head and shoulders. The cameo is even more valuable if the craftsmanship of the image is more ornate or detailed.
The mount of a true antique cameo is made from silver, at least. The more valuable ones are made from gold, although there are some made from base metals that are not going to be of high value. Use the description of the shell, the style of the image and the mount's metal to search online for cameos that resemble yours or are comparable. Check auction house catalogues for past sales of cameo jewelry and any final hammer prices. Contact a reputable jeweler if you are not able to find a comparable cameo to get a valuation.