1. A rounded, more or less hollow concretion or separable nodule of rock lined with inward-projecting crystals which differ in composition from the enclosing rock or the crystal-lined cavity in such a hollowed rock: A geode is a nodule of stone, a roughly spherical rock mass containing a cavity, lined or filled with crystals lining the inside walls, that have grown unimpeded and so are frequently perfectly formed.
2. Etymology: from French géode; from Latin geodes; from Greek geodes, "earthy, earth-like"; from ge-, "earth".