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What Is A Geode?

The mysterious earth-shaped geodes have long challenged geologist to explain how they are formed. Geodes are a variable phenomenon and, therefore, many theories exists to explain how they are created. The term geode is derived from the Greek word Geoides which means "earthlike."
A geode is a sphere shaped rock which contains a hollow cavity lined with crystals. A geode which is completely filled with small compact crystal formations such as agate, jasper or chalcedony is called a nodule. The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid.

How Geodes Are Created
Geodes begin as bubbles in volcanic rock or as animal burrows, tree roots or mud balls in sedimentary rock. Over time, the outer shell of the spherical shape hardens, and water containing silica precipitation forms on the inside walls of the hollow cavity within the geode. The silica precipitation can contain any variety of dissolved minerals, the most common being quartz, but amethyst and calcite are also found.
Over a period of thousands 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.
Each geode is unique in composition and can only be truly discovered when cracked open or cut with a rock saw. The size and formation of crystals and different shades of color within the crystals make each geode special. The rough exterior of the geode gives no indication of the secrets held within its core. The anticipation never fades for those who curiously collect buckets full of round geodes and eagerly expose the secrets of each individual sphere-shaped rock. The most prized contain rare amethyst crystals or black calcites.