The Collaborative International Dictionary of English v.0.48:

Onyx \O"nyx\, n. [L., fr. Gr. ? a claw, finger nail, a veined
   gem. See Nail, and cf. Onycha.]
   1. (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.
      [1913 Webster]

   2. Same as Mexican onyx.
      [PJC]

   3. A deep jet-black color.
      [PJC]

   Onyx marble, Mexican onyx, a banded variety of marble or
      calcium carbonate resembling onyx. It is obtained from
      Mexico in various colors ranging from white to deep black.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Mexican onyx
    n 1: a hard compact kind of calcite [syn: alabaster, oriental
         alabaster, onyx marble, Mexican onyx]