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Description
The word geode is a derivative of Latin and means "earthlike." This is a reference to the rounded or spherical shapes often attributed to geodes, though some geodes can be elongated and non-sphere shaped. The most sought-after geodes have hollow centers, although many are solid objects, with the crystalline growth having filled up all the interior space. The size and shape of the crystals inside a geode depend on two features: the internal symmetric structure of the particular crystal and its relative rate of growth along different directional axes.
Types
Geodes are classified based on the minerals found inside them, with the most common one being quartz. The crystal shape of these minerals can be classified--based on their symmetry--into seven groups of three-dimensional patterns. These are cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic. Further, the lattices in crystals can be primitive (one connection per cell) or non-primitive (multiple connections per cell). Combining the seven shape patterns with the two types of lattices results in the 14 Bravais Lattices, a classification system named after Auguste Bravais, a pioneer in the field of crystal research.
Crystals can also be classified by their physical and chemical properties. The four types of crystals in this system are covalent, metallic, ionic and molecular.
Formation
Geologists have been puzzled over the exact formation process of geodes since their discovery. While many theories have been put forth, there is now a consensus in the scientific community on the basic principles of how geodes form. In their beginning phases, geodes are outwardly growing nodules forming around a nucleus in soft, non-rigid sediment. These are also known as concretions. Eventually, the exterior shells of these concretions are replaced by a crystalline structure. The interiors of these concretions then dissolve, allowing the crystals to grow into the empty space. Crystals begin growing in a process known as nucleation, which can either be unassisted/homogeneous or assisted/heterogeneous.
Colors
The brilliant colors often attributed to geodes are a result of the chemical impurities within crystals. Many gems are in fact simple quartz crystals that have been colored by the environments in which they form.
Searching for Geodes
Geodes are only found on Australia and North and South America. One of the most famous spots for geode collecting is Keokuk, Iowa. Because of its unique and bountiful rock formations, many variations of geodes can be found there.