The inside of a geode. Geodes (Greek γεώδης - geōdēs, "earthlike") are geological rock formations
which occur in sedimentary and certain volcanic rocks. Geodes are essentially rock cavities or vugs
with internal crystal formations or concentric banding. The exterior of the most common geodes is
generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony
deposits. Other geodes are completely filled with crystal, being solid all the way through. These
types of geodes are called nodules.

Source: Wikipedia