Source : Webster's Revised Unabridged Dictionary (1913)

Cameo \Cam"e*o\, n.; pl. {Cameos}. [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.

   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.

   {Cameo conch} (Zo["o]l.), a large, marine, univalve shell,
      esp. {Cassis cameo}, {C. rua}, and allied species, used
      for cutting cameos. See {Quern conch}.

Source : WordNet®

cameo
     n : engraving or carving in low relief on a stone (as in a
         brooch or ring)