Source : Webster's Revised Unabridged Dictionary (1913)

Onyx \O"nyx\, n. [L., fr. Gr. ? a claw, finger nail, a veined
   gem. See {Nail}, and cf. {Onycha}.] (Min.)
   Chalcedony in parallel layers of different shades of color.
   It is used for making cameos, the figure being cut in one
   layer with the next as a ground.

   {Onyx marble}, a banded variety of marble or calcium
      carbonate resembling onyx. It is obtained from Mexico.

Source : WordNet®

onyx
     n : a chalcedony with alternating black and white bands; used in
         making cameos