What Is A Geode? How To Find And Open Geodes - Science Notes

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.” Geodes are popular with collectors and geologists for their beautiful interiors and fascinating formation. 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.

Key Takeaways: Geode

• A geode is a hollow rock lined with crystals or minerals deposited by mineral-rich water.
• Most geodes form in igneous or sedimentary rock through mineral deposition in cavities.
• Common geode minerals include quartz, amethyst, calcite, and celestite.
• You can find geodes in deserts, limestone areas, and volcanic regions.
• Open geodes carefully to preserve the crystal interior, using tools like pipe snap cutters or chisels.
• It’s possible to make crystal “geodes” at home using eggshells, candy, or plaster molds.

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:

• Agate
• Amethyst
• Ankerite
• Aragonite
• Calcite
• Celestite
• Chalcedony
• Dolomite
• Goethite
• Gypsum
• Hematite
• Pyrite
• 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.

Examples of Major Minerals and Their Appearance

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. Ideally, look in regions of past volcanic activity or where limestone dissolves over time. 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.

How to Identify a Geode Before Opening

Before cracking open a potential geode, it helps to know what to look for. Geodes are often nondescript from the outside, but there are several clues that help you distinguish them from ordinary rocks or nodules:

• Shape: Geodes are usually round or oval and often resemble lumpy potatoes or cannonballs. They often appear smoother than surrounding rocks due to weathering.
• Texture: The outer surface is typically bumpy, grainy, or chalky rather than shiny or glassy. Many geodes have a thin layer of chalcedony or a whitish rind.
• Color: While the exterior color varies, common geode surfaces are tan, light gray, or bluish-gray. This color is not always a reliable indicator.
• Weight: Geodes are often lighter than solid rocks of similar size because of their hollow interior. A quick heft comparison can be revealing.
• Sound: Give the rock a gentle shake near your ear. You might hear faint rattling if loose crystals or fragments are inside.
• Test with Light: Some people use a bright flashlight in a dark room to check if light penetrates the outer shell, hinting at an internal cavity.
• Location: Consider where you found the rock. If it’s from a known geode-bearing area, especially volcanic or sedimentary regions, the chances improve.
• Surface Clues: Occasionally, small crystals are visible through cracks or holes in the shell, especially in partially weathered geodes.

If you’re still unsure, local rock and mineral societies, geology departments, or rock shops can often help identify potential geodes before you break them.

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.

1. Use a lapidary saw (if you’re lucky enough to have one). Be advised, the oil may damage some types of crystals within geodes.
2. 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.
3. 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.

1. 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.
2. Grow crystals in a plaster of Paris shell. Let the plaster dry and then fill the hollow with any crystal-growing recipe. Alum or borax crystals are popular choices. Bonus tip: After the plaster dries, paint it with glow-in-the-dark paint. Then, grow the crystals to make a glowing geode.
3. 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

• Make a glow in the dark crystal geode.
• Grow a salt crystal geode.

FAQs About Geodes

Q: Are geodes always hollow inside?A: Not always. While most geodes have a hollow cavity (vug), some fill completely with mineral material over time. A fully filled geode is technically a nodule, not a true geode.

Q: What is the difference between a geode and a nodule?A: A geode has a hollow interior lined with crystals, while a nodule is solid throughout and lacks a cavity.

Q: What types of crystals are found in geodes?A: Quartz and calcite are most common, but geodes may contain amethyst, celestite, pyrite, chalcedony, agate, gypsum, and more.

Q: Are geodes valuable?A: Some geodes are quite valuable, especially if they contain large or rare crystals. However, most are valued more for their beauty and educational interest than for high monetary worth.

Q: Can geodes contain fossils?A: Yes. Occasionally, organic material such as wood, shells, or coral is preserved inside geodes. Some even contain fossil impressions.

Q: Where are geodes most commonly found?A: In the U.S., geodes are common in states like Iowa, Indiana, Missouri, Arizona, California, Utah, and Kentucky. They are also found in Brazil, Mexico, and Namibia.

Q: How long does it take for a geode to form?A: Geode formation takes thousands to millions of years, depending on the size of the cavity and the rate of mineral deposition.

Q: Can I make a geode at home?A: Yes! While you can’t replicate natural geodes, you can grow crystals inside eggshells, candy shells, or plaster molds to mimic geodes. These are fun science projects for kids and adults.

References

• 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.

Related Posts