Source : Webster's Revised Unabridged Dictionary (1913)

Geode \Ge"ode\, n. [F. g['e]ode, L. geodes, fr. Gr. ? earthlike;
   ge`a, gh^, the earth + ? form.] (Min.)
      (a) A nodule of stone, containing a cavity, lined with
          crystals or mineral matter.
      (b) The cavity in such a nodule.

Source : WordNet®

geode
     n : (mineralogy) a hollow rock or nodule with the cavity usually
         lined with crystals