Geode
Geode (jē"ōd) , noun
[French géode, Latin geodes, from Greek {not transcribed} earthlike; ge`a, gh^, the earth + e'i^dos form.]
(a)
(Mineralogy) A nodule of stone, containing a cavity, lined with crystals or mineral matter.
(b)
(Mineralogy) The cavity in such a nodule.