One way of identifying an unopened geode is to know what areas are capable of containing them. Geodes occur in sedimentary and igneous formations, such as limestone and basalt flows. When the parent rocks erode away, the geodes are deposited in the immediate area. An unopened geode has outer cortex that makes it hard to distinguish from other rocks, but it is spherical in shape, ranging from perfectly round to oblong.
Geodes often form in small cavities inside sedimentary rock created by ancient animals or tree roots. The process begins when mineral-rich water flows into the cavity and comes into contact with a piece of limestone or anhydrite. The minerals harden around the piece of rock, and the repeated process of mineralization and hardening forms what later becomes a geode. In igneous rock, geodes form when mineralization and hardening occurs inside cavities created by gas bubbles from the original lava eruption.
Geode crystal formation takes many millions of years to occur and is dependant on several factors. For instance, the types minerals deposited during mineralization and the amount of pressure applied to the geode from other surrounding rock has a large impact on how the geode forms. When the conditions for geode formation are optimal, the crystallization process occurs in the center of the nodule and slowly fills out the geode.