What is a Geode?
The term geode is derived from the Greek word Geoides which means "earthlike." A geode is a sphere or oval shaped rock which contains a hollow cavity lined with crystals. Most geodes have a hollow are partially hollow center with quartz crystals and various mineral formations inside. Some geodes are completely filled with small compact crystal formations such as quartz, agate, jasper or chalcedony and are called nodules. The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid.
Geodes begin as bubbles in volcanic rock, animal burrows, balls of mud or tree roots in sedimentary rock. Over time, the outer shell hardens and water containing silica precipitation forms on the inside walls of the hollow cavity within the geode. The silica precipitation can contain many varieties of dissolved minerals, the most common being quartz, but calcite, amethyst and many others are also found.
Over a period of millions of years, layers of silica cool, forming crystals of different minerals within the cavity. Different types of silica cool at varying temperatures, thus creating layers of different types of mineral crystals.
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