What causes a geode to be hollow or solid?
A geode starts out as basically a cavity or pocket in otherwise solid rock. The cavity walls grow inward as mineral crystals grow from solutions that seep into it. In time, the cavity may become completely infilled with mineral crystal. The geodes that are found on the surface are survivors of weathering which eroded the rock in which they were once emplaced. This is because the geodes are composed of minerals such as quartz which are more resistant to erosion than the rocks which originally surrounded them.
