The Collaborative International Dictionary of English v.0.48:

Chalkstone \Chalk"stone`\, n.
   1. A mass of chalk.
      [1913 Webster]

            As chalkstones . . . beaten in sunder. --Isa. xxvii.
                                                  9.
      [1913 Webster]

   2. (Med.) A chalklike concretion, consisting mainly of urate
      of sodium, found in and about the small joints, in the
      external ear, and in other situations, in those affected
      with gout; a tophus.
      [1913 Webster]

WordNet (r) 3.0 (2006):

chalkstone
    n 1: a deposit of urates around a joint or in the external ear;
         diagnostic of advanced or chronic gout [syn: tophus,
         chalkstone]