The Geode is a Greek word which means “the shape of the earth”. It is an oblong or egg shaped rock that consists of a hollow cavity surrounding with minerals. It is a geological secondary formation that develops in volcanic and sedimentary rocks. It is formed by the process of chemical precipitation, and the origin of the Geode is from the sediment mass. The most common minerals lined internally are Pyrite, Calcite, Millerite, Sphalerite, Celestite, Kaolinite, Dolomite, Barite, Opal, Smithsonite, Chalcedony and Limonite. Along with that, the most commonly found minerals in the Geode are the macro-crystalline form of quartz. Geode rock is mostly found in limestone and basaltic lavas.
The Geode is initially a hollow bubble formed in the volcanic rock. As time passes, the outer shell of the bubble gets hardened. The inner walls of the cavity present in the Geode are formed by the water flow that consists of silica precipitation. The precipitation can possess any type of dissolved minerals, but some commonly found ones are amethyst, quartz and calcite. Later, after a period of thousands of years, the silica layer cools inside the cavity along with other minerals, thus forming different layers of minerals in the hollow cavity.
The other way in which Geode rocks form is by the development of a bubble in animal burrow or tree root. The hollow space is left by the animals when they move on, or by the trees, when they die and the roots get rotted. That hollow space is covered with sediments hundreds of feet thick accumulated over thousands of years. Over the time, the weight of the layers over the sediment causes it to transform into a rock or a sedimentary rock. Then, similar to the formation of the Geode rock in the volcanic rock, this bubble is also filled with a mineral solution inside out. After the solution cools down, a wide variety of minerals are found clinging around the bubble, which later turn into layers of minerals in the cavity of the Geode rock.
Every Geode is different in its features depending on the environment where it was formed, and on the minerals which compose them. The features cannot be discovered by checking the rough and hard exterior, but only by opening or cutting the Geode using a rock saw. The size, color and the appearance make each and every Geode unique. The color and banding depends on the variable impurities in the silica solution. The presence of Iron ore in the silica water will give a rust color to the Geode rock. The other types have amethyst crystals, a purple hue. Geode rocks acquired from the same location are generally similar in hues.
The Geodes are very commonly seen in the United States, Namibia, Brazil and Mexico. Some formations in places like Missouri, Utah, Indiana, Kentucky and Iowa consist of Geode rocks. The largest Geode was found in an abandoned cave situated in the silver mine at Almeria, Spain, which has a cavity as big as 1.8 x 1.7 m and a length of 8 meters. The cave’s entrance is occluded by more than 5 tons of Geode rocks. Some geologists consider that the cave was formed during the Messinian salinity crises that took place about 6 million years back. It is believed that at that time, the Mediterranean Sea evaporated and thick layers of evaporites or salt sediments were left behind, out of which these unique rocks have emerged. Large size Geode is usually seen in Ohio located on the South Bass Island.