What is a cameo?

A cameo is an engraved gem on which the subject or figure is carved in relief on a stone or other material which provides two layers of color, such as chalcedony, onyx and shell. One color forms the background while the design is cut out of the other. Finished cameos are often mounted as brooches. Favourite subjects are portraits and mythological scenes. Among the portraits are valuable representations of Roman emperors and princes.