What in the World is a Geode?

A geode is a type of rock formation that occurs when a pocket of air or gas is trapped and forms a cavity in sedimentary or igneous rock. A hard “crust” forms around the outside of the cavity. Groundwater, which often carries dissolved minerals in solution, seeps through the rock over a long period of time. The result? Beautiful crystal points or layers of banded crystals deposited inside the cavity – much to the wonder of the lucky person who gets to be the first ever to peek inside!

Like a cave, the mineral rich solution that leaves crystals behind inside a geode is a naturally occurring process between groundwater and the surrounding rock. Crystal size, color and chemical composition will vary and each individual geode is unique. Clear quartz, smoky quartz, amethyst, calcite, celestite, and chalcedony are examples of some minerals you may find inside a geode. Geodes from the same location often have the same type of mineral crystals. Some geodes are filled in with beautiful layers of mineral crystals. These are sometimes dyed brilliant colors and cut or sliced for decorative use.

Brazil is one place in the world that has some amazing geodes! Amethyst cathedral geodes can range in size from a few inches tall to the size of a small vehicle! Citrine, also called burnt amethyst, also forms in large geodes. Amethyst and Citrine are both a type of quartz.

Cave of the Mounds National Natural Landmark is proud to offer a wide selection of geodes from around the world in our amazing rock & fossil shop. You can take it home and break it yourself, or we’ll use one of our special machines to break it for you. Regardless, you never know what you are going to get – but guaranteed, you’ll be the first to discover the inside of your own unique geode specimen!

To purchase a Break-Your-Own-Geode visit our online store.

Links to some awesome sites where you can learn more: