Geodes, a term derived from a Greek word meaning earth-shaped, are irregular, roughly spherical bodies. They can be oblong or shaped like invertebrate fossils. Some are hollow and lined with most beautiful and unusual layers andclusters of various mineral crystals, but others are completely filled by inward-growing crystals.geode

Hollow geodes, relatively lightweight compared with those completely filled, are more desirable because they generally contain a greater variety of minerals that have grown well formed crystals.