Geode

from WordNet (r) 3.0 (2006)

geode
    n 1: (mineralogy) a hollow rock or nodule with the cavity
         usually lined with crystals

from The Collaborative International Dictionary of English v.0.48

Geode \Ge"ode\ (j[=e]"[=o]d), n. [F. g['e]ode, L. geodes, fr.
   Gr. ? earthlike; ge`a, gh^, the earth + e'i^dos form.] (Min.)
      (a) A nodule of stone, containing a cavity, lined with
          crystals or mineral matter.
      (b) The cavity in such a nodule.
          [1913 Webster]