A geode is a sphere shaped rock which contains a hollow cavity lined with crystals. Geodes are created in the hollow areas of soil such as animal burrows or tree roots. They are also formed in the bubbles in volcanic rock. Over time, dissolved minerals seep into a hollow area and harden into an outer shell creating the geode. They look uninteresting on the outside, but slicing them in half may reveal highly attractive patterns and colors valuable in jewelry.

Keokuk is a city in the southeastern part of Iowa where geodes can be found in abundance. Considered the Holy Grail of all Keokuk geodes is a very hollow iridescent black calcite geode. It has flashes of green when turned in the light. It is the rarest of all Keokuk geodes. Another museum quality geode is the brown calcite keokuk geode pair. There are clear and brown calcite cubes in this one and creates very brilliant flashes of greens and purples throughout the interior.