The Collaborative International Dictionary of English v.0.48:

dolomite \dol"o*mite\ (d[o^]l"[-o]*m[imac]t), n. [After the
   French geologist Dolomieu.] (Geol. & Min.)
   A mineral consisting of the carbonate of lime and magnesia in
   varying proportions. It occurs in distinct crystals, and in
   extensive beds as a compact limestone, often crystalline
   granular, either white or clouded. It includes much of the
   common white marble. Also called bitter spar.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dolomite
    n 1: a kind of sedimentary rock resembling marble or limestone
         but rich in magnesium carbonate
    2: a light colored mineral consisting of calcium magnesium
       carbonate; a source of magnesium; used as a ceramic and as
       fertilizer [syn: dolomite, bitter spar]