The Collaborative International Dictionary of English v.0.48:

Zircon \Zir"con\, n. [F., the same word as jargon. See Jargon
   a variety of zircon.]
   1. (Min.) A mineral consisting predominantly of zirconium
      silicate (Zr2SiO4) occurring in tetragonal crystals,
      usually of a brown or gray color. It consists of silica
      and zirconia. A red variety, used as a gem, is called
      hyacinth. Colorless, pale-yellow or smoky-brown
      varieties from Ceylon are called jargon.
      [1913 Webster +PJC]

   2. an imitation gemstone made of cubic zirconia.
      [PJC]

   Zircon syenite, a coarse-grained syenite containing zircon
      crystals and often also elaeolite. It is largely developed
      in Southern Norway.
      [1913 Webster]

WordNet (r) 3.0 (2006):

zircon
    n 1: a common mineral occurring in small crystals; chief source
         of zirconium; used as a refractory when opaque and as a gem
         when transparent [syn: zircon, zirconium silicate]