A geode is a hollow rock containing minerals or crystals deposited by mineral-rich water. The word “geode” comes from the Greek word for “Earth-like.” Here is a look at how geodes form, colors and types of crystals they contain, how to find geodes, how to crack them open, and how to make them yourself.
How Geodes Form
Geodes form when mineral-laden water fills spaces in igneous or sedimentary rock. The rock gets spaces from gas bubbles in igneous rock or by tectonic shifts forming cracks in sedimentary rock. The minerals forming the geode come from ground water or hydrothermal fluids.
Over time, minerals deposit from the dissolved silicates and carbonates in water. Geodes contain hollow spaces called vugs. Vugs range in size from a few millimeters to crystal caves. If the original rock fills completely with minerals, it’s called a nodule.
Many geodes are rounded rocks. What happens is that the crust lining the void inside a rock is a tough mineral, like chalcedony or amethyst. Eventually, the original rock around the hard shell wears down, freeing the tough mineral, which in turn contains other minerals or crystals.
Types of Crystals in Geodes
Geodes come in many colors and may contain any of several minerals or crystals. Typically, these minerals are silicates or carbonates. Minerals include:
- Rock crystal (quartz)
- Rose quartz
- Smoky quartz
Sometimes water flowing into a space carries diatoms, coral, sponges, wood, and other organic material. Some geodes contain fossils, coal, or even liquid petroleum.
How to Find Geodes
Geodes are available for purchase in stores and online, but you can find them yourself. First, look in the right place. Seek deserts, areas rich in limestone, and volcanic ash beds. In forested areas, check river beds and lake shores. In the United States, geodes are common in Arizona, California, western Illinois, Indiana, Iowa, Kentucky, Missouri, Nevada, and Utah. The are abundant in Brazil, Mexico, and Namibia. In Somerset, England geodes are common enough to have their own name (“potato stones”).
Wherever you go, look for a rounded rock. While the minerals within geodes may be any color, usually the rock is bluish-gray or tan. If you’re not sure where to look or how to identify a geode, contact the local gem and mineral society or search online for nearby locations.
How to Open a Geode
There’s more than one way to crack open a geode. But, some methods break the geode into fragments.
Safe Way for Kids to Open Geodes
- Place the geode in a sock over wrap it in a dish towel. This prevents pieces of rock from flying off and causing injuries.
- Safety glasses and gloves are a good idea, but not essential if the geode is covered.
- Strike the geode with a hammer or even a second rock. A rock hammer works best, but you can use whatever you have. Don’t smash the geode with all your might. You only want to break it open.
How to Open a Geode Without Smashing It
The problem with the sock-and-hammer method is that the sock distributes the force of the hammer. The increased surface area means it’s more likely the geode breaks in several pieces. It’s better to cut the geode or strike strategically. For all these methods:
- Wear gloves to protect your hands from shard.
- Wear safety goggles to protect your eyes.
- Either cut the geode wet or else wear a dust mask.
- Use a lapidary saw (if you’re lucky enough to have one). Be advised, the oil may damage some types of crystals within geodes.
- Use a pipe snap cutter. This is a common plumber’s tool that you wrap around the geode like a bicycle chain. Once the chain is tight, apply pressure using the handle and snap the geode in two. This method works well for geodes that are the appropriate size for the tool.
- Using a hammer, lightly tap a flat masonry chisel and score the geode. Rotate the geode slightly and tap a new line. Proceed until you work your way around the geode. Scoring the geode controls where it breaks. If the geode doesn’t break when you complete a circuit around it, repeat the process until it does. This method isn’t as quick as using a saw or pipe snap cutter, but it’s effective.
How to Make a Homemade Geode
Natural geodes take millions of years to form, but you can make homemade versions much faster. Basically, you grow crystals within some type of outer shell.
- Fill a hollow chocolate or candy shell with sugar crystal solution. Either use a candy mold to make the geode shell or else fill a store-bought holiday hollow chocolate (like a hollow Santa or Easter Bunny). Allow about a week for crystal growth. Then, drain the remaining liquid, leaving only the sugar crystals. This geode is edible.
- Grow crystals in a plaster of Paris shell. Let the plaster dry and then fill the hollow with any crystal-growing recipe. Bonus tip: After the plaster dries, paint it with glow-in-the-dark paint. Then, grow the crystals to make a glowing geode.
- Grow crystals in an egg shell. Break open an egg shell, eat or discard the egg, and clean the shell. Remove the membrane lining the inside of the egg. Fill the shell with any crystal recipe. After the crystals grow, pour off the excess liquid and enjoy the geode.
Projects to Try
- Middleton, Gerard V. (2003). Encyclopedia of Sediments and Sedimentary Rocks. Springer. ISBN 978-1-4020-0872-6.
- Pough, Frederick H.; et al. (1998). A Field Guide to Rocks and Minerals (5th ed.). ISBN 0-395-91096-X.
- Wolfe, Mark E. (May 2014). Celestine in Ohio. Ohio Department of Natural Resources.