What Is the Definition of a Geode?
According to Princeton University's WordNet database, a geode is a hollow rock or nodule with the cavity usually lined with crystals. Although a majority of these crystals are quartz, some may contain amethyst and calcite.
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Hollow Cavity
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The hollow cavities inside geodes are usually formed by bubbles or air within volcanic rock, from an animal burrow or from tree roots.
Crystal Structure
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Over thousands of years, the geode will harden and mineral-rich water will mix with the mineral crystals inside the hollow cavity. These crystals form along the inside wall.
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Location
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Geodes are usually found in desert areas, volcanic ash beds and areas with abundant limestone. In the U.S., they are common in California, Arizona, Utah, Nevada and even Iowa.
Coloration
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The color of geode crystals comes from the different types of minerals that have crystallized inside. It's not uncommon to see multiple bands or rings of reds, browns, blues and pinks. Some may be a solid, single color.
Heat
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Heat also affects coloration. For example, when an amethyst loses heat, the normal purple color can become yellow or citrine.
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