The Collaborative International Dictionary of English v.0.48:

Limestone \Lime"stone`\ (l[imac]m"st[=o]n`), n.
   A rock consisting chiefly of calcium carbonate or carbonate
   of lime. It sometimes contains also magnesium carbonate, and
   is then called magnesian or dolomitic limestone. Crystalline
   limestone is called marble.
   [1913 Webster]

WordNet (r) 3.0 (2006):

limestone
    n 1: a sedimentary rock consisting mainly of calcium that was
         deposited by the remains of marine animals