GEODE

n.

Hollow mineral body found in limestones and some shales, commonly a slightly flattened globe 1–12 in.

(2.5–30 cm) in diameter and containing a chalcedony layer surrounding an inner lining of crystals. The hollow interior often is nearly filled with inward-projecting crystals, new layers growing on top of old. The crystals are often of quartz but sometimes of other minerals.