A geode is a rock with a special surprise inside. From the exterior, a geode appears to be a boring round-shaped rock with little to capture one's attention. If a rock like this is broken open, however, it will reveal a center that is filled with colorful crystals. "Geo" means earth and these amazing rocks are typically round in shape like a planet.
Geodes are created when air pockets form in hot volcanic rock. Over time, the crystals gradually form as water seeps in and out of the air pocket. The minerals contained in the water get left behind and these minerals gradually build on each other to create the crystals.
The way the crystals form and the number of crystals that form are different for every geode. Sometimes the crystals are smooth and may fill the center space completely. Other times the crystals are jagged and rough and only a portion of the center space will have crystals formed.
Geode crystals look different depending on the location in which they are formed. The colors of the crystals are largely determined by the kinds of minerals present in the water that seeps into the air pockets. Some of the most striking geodes are found in South America. These geodes have dark purple crystals in the centers and are called amethyst geodes.
A very large geode was discovered in 1999 in Spain. This geode is 26 feet long. The crystals in the center are white and some are over three feet long. It is hypothesized that the crystals formed millions of years ago when the Mediterranean Sea evaporated a great deal and left salt in the geode air pocket that eventually became the crystals.
Brilliant geodes can often be sold for large sums of money. Smaller geodes that are less significant are exciting finds, nonetheless, and can make attractive paperweights or interesting knickknacks on a shelf.