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
   origin.]
   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.
         [1913 Webster]

   Cameo conch (Zool.), a large, marine, univalve shell, esp.
      Cassis cameo, Cassis rua, and allied species, used for
      cutting cameos. See Quern conch.
      [1913 Webster]