Inside and outside views of the geode. Geodes (Greek Î³ÎµÏÎ´Î·Ï - ge-ÅdÄs, earthlike) are geological secondary structures which occur in certain sedimentary and volcanic rocks.ost geodes contain clear quartz crystals, while others have purple amethyst crystals. Still others can have agate, chalcedony, or jasper banding or crystals such as calcite, dolomite, celestite, etc. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart.