The Collaborative International Dictionary of English v.0.48:

Onycha \On"y*cha\, n. [NL., from L. onyx, -ychis, onyx, also, a
   kind of mussel, Gr. ?, ?. See Onyx.]
   [1913 Webster]
   1. An ingredient of the Mosaic incense, probably the
      operculum of some kind of strombus. --Ex. xxx. 34.
      [1913 Webster]

   2. The precious stone called onyx. [R.]
      [1913 Webster]

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]

The Collaborative International Dictionary of English v.0.48:

onyx \o"nyx\, a.
   Deep, jet-black.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Chalcedony \Chal*ced"o*ny\ (k[a^]l*s[e^]d"[-o]*n[y^] or
   k[a^]l"s[-e]*d[-o]*n[y^]; 277), n.; pl. Chalcedonies
   (-n[i^]z). [ L. chalcedonius, fr. Gr. CHalkhdw`n Chalcedon, a
   town in Asia Minor, opposite to Byzantium: cf. calc['e]doine,
   OE. calcidoine, casidoyne. Cf. Cassidony.] (Min.)
   A cryptocrystalline, translucent variety of quartz, having
   usually a whitish color, and a luster nearly like wax.
   [Written also calcedony.]
   [1913 Webster]

   Note: When chalcedony is variegated with with spots or
         figures, or arranged in differently colored layers, it
         is called agate; and if by reason of the thickness,
         color, and arrangement of the layers it is suitable for
         being carved into cameos, it is called onyx.
         Chrysoprase is green chalcedony; carnelian, a flesh
         red, and sard, a brownish red variety.
         [1913 Webster]

WordNet (r) 3.0 (2006):

onyx
    n 1: a chalcedony with alternating black and white bands; used
         in making cameos