Geodes are geological rock formations. They occur in both sedimentary and certain types of volcanic rocks. Geodes are vugs (a small cavity or hollow space inside a rock). They have concentric banding which means they share a common center. They also have internal crystal formations.
Most common geodes have an exterior made up of limestone or a similar rock. The interior of the geode has chalcedony deposits and/or quartz crystals. Chalcedony are translucent or transparent milky examples of quartz. Some geodes are filled entirely with crystal and are completely solid. These geodes are called nodules.
Geodes are usually the rounded in formations and are found in sedimentary and igneous rocks. Gas bubbles in igneous have been known to contain geodes. The geode forms when the rock encircling the cavity hardens. Carbonates and/or silicates are deposited on the inside surface. Gradually, minerals from groundwater or hydrothermal solutions helps crystals form in the cavity or hollow space.
Some of the most unusual places that geodes have been known to form, are animal burrows, mud balls, or tree roots. Almost anywhere a hollow space can be found, a geode can be formed if given the right circumstances.
Rocks that contain geodes wear away and decompose over time. This allows the geodes to surface because the quartz material from which they are made is extremely hard and resilient.
While a few geodes contain purple amethyst crystals, most of them have clear quartz crystals. A few contain chalcedony, agate, jasper banding, or crystals such as domomite, celestite, calcite and others. While it is impossible to be sure of what type of geode may be in a particular rock, similar geodes are usually found in the same areas.
The United States, Brazil, Mexico and Namibia are where most geodes are found. Geodes are found in desert regions more than anywhere else. A large geode was found in Ohio in the early 20th century and is now a tourist attraction. It is knows as Crystal Cave. The geode is known in Iowa as the official rock. Iowa also has a Geode State Park.
The largest geode ever found is located near Almeria, Spain. It was found in 2000. It measures 1.8 metres by 1.7 meters. The front of the cave is blocked by five tons of rocks. To prevent looters from going inside and picking up the crystals the cave is under police protection. it is not open to the public.