Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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