Cameo jewellery is often seen as brooches, rings and earrings. Less common but just as attractive are cameo charms, I suppose that these are found less often as a charm is small and therefore the picture is difficult to carve in miniature. Cameos are carved so that the picture stands proud of the background. Often but not essentially the design is in a contrasting colour to the background.
Cameos can be made from all sorts of materials, normally we think of shell cameos as can be seen in this cameo charm: