The Collaborative International Dictionary of English v.0.48:

Topaz \To"paz\, n. [OE. topas, F. topaze, L. topazos, or
   topazion, a kind of precious stone, Gr. to`pazos, topa`zion;
   possibly akin to Skr. tap to glow (cf. Tepid). According to
   some, the name is from Topazos, a small island in the Red
   Sea, where the Romans obtained a stone which they called by
   this name, but which is the chrysolite of the moderns.]
   1. (Min.) A mineral occurring in rhombic prisms, generally
      yellowish and pellucid, also colorless, and of greenesh,
      bluish, or brownish shades. It sometimes occurs massive
      and opaque. It is a fluosilicate of alumina, and is used
      as a gem.
      [1913 Webster]

   2. (Zool.) Either one of two species of large, brilliantly
      colored humming birds of the genus Topaza (Topaza
      pella or Topaza pyra), of South America and the West
      Indies.
      [1913 Webster]

   Note: The two tail feathers next to the central ones are much
         longer that the rest, curved, and crossed. The Throat
         is metallic yellowish-green, with a tint like topaz in
         the center, the belly is bright crimson, the back
         bright red. Called also topaz hummer.
         [1913 Webster]

   False topaz. (Min.) See the Note under Quartz.
      [1913 Webster]

WordNet (r) 3.0 (2006):

topaz
    n 1: a yellow quartz [syn: topaz, false topaz, common
         topaz]
    2: a mineral (fluosilicate of aluminum) that occurs in crystals
       of various colors and is used as a gemstone
    3: a light brown the color of topaz [syn: tan, topaz]

Easton's 1897 Bible Dictionary:

Topaz
   Heb. pitdah (Ezek. 28:13; Rev. 21:20), a golden yellow or
   "green" stone brought from Cush or Ethiopia (Job 28:19). It was
   the second stone in the first row in the breastplate of the high
   priest, and had the name of Simeon inscribed on it (Ex. 28:17).
   It is probably the chrysolite of the moderns.