The Collaborative International Dictionary of English v.0.48:

Calcite \Cal"cite\ (k[a^]l"s[imac]t), n. [L. calx, calcis,
   lime.] (Min.)
   Calcium carbonate, or carbonate of lime. It is rhombohedral
   in its crystallization, and thus distinguished from
   aragonite. It includes common limestone, chalk, and marble.
   Called also calc-spar and calcareous spar.
   [1913 Webster]

   Note: Argentine is a pearly lamellar variety; aphrite is
         foliated or chalklike; dogtooth spar, a form in acute
         rhombohedral or scalenohedral crystals; calc-sinter and
         calc-tufa are lose or porous varieties formed in
         caverns or wet grounds from calcareous deposits; agaric
         mineral is a soft, white friable variety of similar
         origin; stalaclite and stalagmite are varieties formed
         from the drillings in caverns. Iceland spar is a
         transparent variety, exhibiting the strong double
         refraction of the species, and hence is called doubly
         refracting spar.
         [1913 Webster]

WordNet (r) 3.0 (2006):

calcite
    n 1: a common mineral consisting of crystallized calcium
         carbonate; a major constituent of limestone