A geode is a hollow geological rock formation found in volcanic or sedimentary rocks. Crystals form inside rock cavities or vugs. Many geodes are spherical in shape. A vug forms along fissures, rather than the spherical formation. The exterior is commonly limestone and the inner crystal formations are quartz or chalcedony deposits. Some geodes, called nodules, are solid minerals, without being hollow. They come in many different sizes, as well as different and unusual shapes.
Cavities are produced by steam in volcanic rock. In other places, they are formed by limestone or anhydrite nodules that washed away, creating a hollow for minerals to deposit in. Dissolved silicates and/or carbonates from groundwater or hydrothermal solutions deposit themselves on the inside surface. Over time, these deposits build themselves into crystals on the surface of the inside chamber. Slower deposits create crystals. Faster ones create bands of finely grained crystals, like agate. Geodes are exposed as the bedrock containing them erodes. As a result, they can often be found in riverbeds. It is estimated that it can take 240 million years to create a geode.
Another way geodes have formed is in the remains of animal burrows or cavities in which ancient tree roots have rotted away. These artificially created hollows hardened over time. Minerals seeped in to create the inner mineral deposits. This accounts for how unusual shapes, including “whale-tail”, have formed.
Most contain clear quartz. Some contain purple amethyst. Some have banding consisting of agate, chalcedony or jasper. Still others contain calcite, dolomite, celestite, citrine, etc. The interior mineralization is hard to identify without breaking the rock apart. However, the minerals can be guessed at from where it was found, as the location tends to restrict the type of mineral present. Geodes can also be synthetically altered to produce different colors or formations. Unusual coloration or formation is a sure way to tell if the rock has been synthetically altered.
Geodes are commonly found across the North Americas. A large geode, known as Crystal Cave, was discovered in Put-In-Bay, Ohio. It contains celestite crystals. This cave is accessible to tourists during the summer. However, the largest geode ever found is in an abandoned silver mine in Almeria, Spain. It measures 8 meters in length, 1.8 meters wide and 1.7 meters high. It contains giant gypsum crystals. Some of these crystals are three feet or more in length. This cave is protected from looters by five tons of rocks and the police. It is not accessible to tourists.