ETYM French géode, Latin geodes, from Greek gea the earth + eidos form.
Cavity in stone lined with crystals.
1. A nodule of stone, containing a cavity, lined with crystals or mineral matter.
2. The cavity in such a nodule.
In geology, a subspherical cavity into which crystals have grown from the outer wall into the center. Geodes often contain very well-formed crystals of quartz (including amethyst), calcite, or other minerals.