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):

chalcedony
    n 1: a milky or greyish translucent to transparent quartz [syn:
         chalcedony, calcedony]

Easton's 1897 Bible Dictionary:

Chalcedony
   Mentioned only in Rev. 21:19, as one of the precious stones in
   the foundation of the New Jerusalem. The name of this stone is
   derived from Chalcedon, where it is said to have been first
   discovered. In modern mineralogy this is the name of an
   agate-like quartz of a bluish colour. Pliny so names the Indian
   ruby. The mineral intended in Revelation is probably the Hebrew
   _nophekh_, translated "emerald" (Ex. 28:18; 39:11; Ezek. 27:16;
   28:13). It is rendered "anthrax" in the LXX., and "carbunculus"
   in the Vulgate. (See CARBUNCLE.)