Jason, Here is a primer on geodes. Geodes tend to form in two geologic environments, sedimentary rock, and extrusive lave flows and ash beds. In sedimentary rock, openings can be created several ways, but they all relate to groundwater dissolving away a hole on the rock. That same ground water carries dissolved minerals, mostly quartz and calcite, and fills the pocket with crystals. Geodes are different from crystal lined vugs or pockets in that the entire pocket gets lined with a layer of microcrystaline material, then larger crystals can form inside.
Geodes can also form in gas formed pockets in cooling volcanic ash beds and lavas.
They are characterized by an outer layer of agate, with a crystal lined center, usually quartz. The geodes of the Sierra Gallegos region of Chihuahua, Mexico are some of the better known geodes of this type. They are typically found in volcanic ash beds. The amethyst geodes of the Serra Geral basalt flows of southern Brazil and Uruguay form in pockets in the basalt. Some of the very long, thin geodes known as cathedrals appear to have formed in gas vents.