GEODES are spheroidal to eccentric-shaped rocks formed when minerals either enter or are trapped in openings or voids inside preexisting rock material. In volcanic rocks, fluids and gases often remain inside air pockets in the lava. During and after cooling, these fluids, from the outside in, fill the cavity with various successions of agate and quartz layers, often ending in open cavities with inward-facing quartz crystals. Other geodes may be formed by cavities formed by dissolved organic material, fractures in the rock, or by pockets caused by dissolved mineral material. These geodes will often have tapered layers that lead to in "inlet", where the filling solutions entered the cavity.
Rarely, a geode will be partially filled in the center by a horizontal sequence of layers inside the concentric bands, ending in a flat surface. This is a gravity filling of the cavity with materials probably under less pressure, ending when there are no more fluids. You can tell which way was "up" when the geode formed.
Colors within the geodes are caused from elements, such as iron, that are carried in the solutions. Most geodes are composed of agates, a member of the quartz family, although some geodes are formed of calcite, usually in limestone pockets.
Composition may include:
Quartz crystals, amethyst crystals, agate, chalcedony, jasper, calcite, dolomite, celestite. Prevalence: U.S., Brazil, Namibia, Mexico.