Geodes are rounded, generally hollow masses that occur mostly in
limestones. They are scattered through the rocks and can be lifted or
dug out. Some geodes are as small as walnuts, and some are as large as
basketballs. Most of them have a rough, dull-looking outer surface. If
you break geodes open, you will find that many are lined with beautiful
crystals of calcite, celestite, or quartz that point inward toward the
hollow center.
[Illustration: Calcite geode found in Lower Cretaceous strata of western
Travis County, Texas.]
It is thought that a geode forms when water, carrying dissolved mineral
material, seeps into a cavity in the rock, then deposits the mineral
material as a lining in the cavity. This lining becomes the outer part
of the geode. Thus a geode—unlike a concretion, which grows from the
center outward—forms from outside to inside.
Some of the Lower Cretaceous limestone rocks of Travis, Williamson, and
Lampasas counties contain calcite and celestite geodes. Celestite geodes
have also been found in Permian rocks in parts of Coke, Fisher, and
Nolan counties.