The term "geode" comes from the Greek word "geoides," which means "earth-like". A geode is a round rock that contains a hollow cavity lined with crystals. Geodes begin as bubbles in volcanic rock, and over time the outer shell hardens and water containing silica precipitation forms on the inside walls. The silica precipitation contains a variety of dissolved minerals, but the most common is quartz. Over a period of thousands of years, layers of silica cool to form beautiful crystals.