geode (n.)

rounded stone with a hollow center lined with crystals, 1670s (in Greek form from 1610s), from French géode, from Latin geodes, name of a certain precious stone, from Greek geodes "earthy, earth-like, with deep soil," from ge "earth" (Homeric gaia; see Gaia) + -oides, adjective suffix, "characterized by" (see -oid). Perhaps so called in reference to the rough crust in which the crystals are hidden. Related: Geodic.