Geodes are geological secondary structures which occur in certain sedimentary and volcanic rocks. They are themselves of sedimentary origin formed by chemical precipitation.
A geode typically begins when a cavity forms in a rock, which can happen several ways.
Cavities are most common in igneous rock created by cooling lava or magma. Usually this happens when a bubble of carbon dioxide and water vapor forms in flows of lava.
Geodes can also form in sedimentary rocks such as limestone or sandstone. The cavity in these rocks is usually formed from a solid core. A mass of minerals, or nodule, in the sediment may begin to dissolve and leave space behind. In other cases, organic matter such as coral, a fossil, or a piece of wood buried in the sediment weathers out over time.
This geode is about an inch in diameter and is on a 22 inch sterling silver chain.