The Collaborative International Dictionary of English v.0.48:
Cameo \Cam"e*o\ (k[a^]m"[-e]*[-o]), n.; pl. Cameos
(k[a^]m"[-e]*[=o]z). [It. cammeo; akin to F. cam['e]e,
cama["i]eu, Sp. camafeo, LL. camaeus, camahutus; of unknown
A carving in relief, esp. one on a small scale used as a
jewel for personal adornment, or like.
[1913 Webster +PJC]
Note: Most cameos are carved in a material which has layers
of different colors, such stones as the onyx and
sardonyx, and various kinds of shells, being used. The
classical cameos made in Italy are carved on a seashell
(see cameo conch, below), having an olive figure
carved from the inner layer of the shell in relief on
the white background of the outer layer of the shell.
Cameo conch (Zool.), a large, marine, univalve shell, esp.
Cassis cameo, Cassis rua, and allied species, used for
cutting cameos. See Quern conch.