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Method of Formation
Scientists believe that geodes form through two main processes. Roddy Sue Behmer, M.Ed., a retired earth science teacher, describes the first process as being similar to fossilization. She says that over time, minerals deposited by water will seep into the small cavities of a porous rock. As the water dries, the minerals form crystals. The second method of formation happens most frequently in limestone caves with subterranean rivers. In this situation, water deposits minerals directly into a crack in a rock. Over millions of years, the minerals fill the crack and the cavity forms a crystal lining.
Size and Shape
You can also categorize geodes by their size and shape. Most geodes fall between 5 to ten centimeters across. On occasion, however, you may encounter an especially large geode. One monster specimen from Spain is 25 feet long and 6 feet wide. Most geodes have a roughly spherical or egg shape. However, some geodes have a more oblong shape. These are commonly known as cathedral geodes.
Inside Minerals
When you break open a geode, you'll most likely see quartz or calcite crystals. However, many geode crystals have variations in color stemming from different trace minerals that leached into the crystals during their formation. For example, magnesium lends a purple hue to quartz and a pinkish hue to calcite. Some geodes have crystals made primarily from fluorite. Lumpish in appearance, these geodes glow under a blacklight.
Place of Origin
Geodes develop with crystals that represent the mineral deposits of the area in which they're found. Keokuk geodes are one example. Found only near Keokuk, Iowa, these geodes contain deposits of 17 different minerals, with quartz being the most common crystal. The fame of the Keokuk geode led the Iowa Legislature to adopt it as the state rock. Amethyst geodes, however, generally hail from Brazil and other places in South America.