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Cavity Formation
The cavity in which the geode forms can be created in a number of ways, depending on the rock type. Igneous rocks, which are formed by molten lava, can sometimes contain bubbles that go on to form geodes. Sedimentary rocks are formed from earth or sand subjected to high pressures. Geodes in sedimentary rocks can form from animal burrows or tree roots.
Crystal Formation
The crystals form in the cavities because of a lengthy process of water deposition and evaporation. Water flows or leaches into the cavity then evaporates, and chemicals in the water are deposited. These deposits form the crystals. Many different types of crystals can form geodes. These crystals include chalcedony, jasper, agate and quartz. Chalcedony is a very hard, fine-grained type of quartz that forms the hard outer covering of most geodes.
Coloration
The color of the crystals in a geode are determined by the chemical impurities present in the crystals. These in turn come either from the rock in which the geode has formed, or from the water that was in contact with the geode during its formation.
Geode Dying
There are two basic methods for artificially changing the colors of geodes. These are by adding more chemical impurities or by heating the geode. By immersing geodes in a watery solution of magnesium and calcite the geode crystals will turn pink. Exposure to heat will turn the purple amethyst geodes yellow.