Jumping For Geodes: Can You Tell the Inside from the Outside?
AbstractHave you ever heard the expression, "You can't judge a book by its cover"? What do you think that means? That a book with a very plain cover might have a very exciting and interesting story inside? Well, in this geology science project, you'll see if the same expression holds true for a rock, but not just any old rock, a special type of rock called a geode, which looks rather plain and ordinary on the outside, but inside can hold crystals and beautiful colors! You'll discover if the texture or color of the outside helps you determine what the inside will look like when you break it open.
ObjectiveTo determine if it is possible to predict the size of a geode's crystals and its inside color from its texture and color on the outside.
Kristin Strong, Science Buddies
Last edit date: 2017-11-17
Do you like surprises, like taking the wrapping paper off a present and seeing what is inside? Or biting into a chocolate and finding your favorite filling on the inside? Well, Earth has surprises, too, in the form of special rocks called geodes. The word geode comes from the Greek word for "Earth," because people thought that unopened geodes looked like little brown or gray Earths.
Geodes are sphere-shaped rock formations that look very plain and ordinary on the outside, but if you break into their shells, you find that they are often hollow inside and filled with beautiful crystals and colors. Crystals have many flat sides to reflect light and that gives them their sparkle. The most common crystal you can find on the inside of a geode is quartz, a mineral. Minerals are the building blocks of rocks.
Figure 1. The crystals in this photo are made up of quartz, a mineral. (Wikimedia Commons, 2008.)
Quartz comes in many different colors, but when it is a violet color, it is called amethyst. Geodes containing amethyst inside are rarer and more valuable.
Figure 2. This photo shows an example of amethyst, a violet form of quartz. (Wikimedia Commons, 2009.)
Geodes are formed inside two types of rock: volcanic rock (from volcanoes), and sedimentary rock, a type of rock that is formed over millions of years when layer after layer of sand, shells, plants, and tiny animal bodies are squeezed together until they are hard. Geodes are created inside the spaces inside these two types of rocks. In volcanic rock, they form inside gas bubbles. In sedimentary rock, they form inside hollows or cavities, like those made by a tree root, animal, or a mud ball. Around this hollow space, a shell forms and hardens. If water containing tiny pieces of minerals slowly fills up the space beneath the shell, then crystals can form inside. It can take millions of years for the crystals to grow and the space to fill up. Most geodes are not entirely filled with crystals, but if they are completely solid, then they are called nodules.
Geodes can be just a few inches across or several feet long. They can be spherical, like a basketball, or more oblong, like an egg. Geodes can be found all over the world, especially in deserts or in areas where there were once volcanoes. In the United States, you'll find them in many states, like California, Arizona, Utah, Nevada, Illinois, Missouri, Kentucky, and Iowa. In fact, the beautiful geode is Iowa's state rock.
Wherever you find geodes, or whatever their shape or size, it is always exciting to find out if they have sparkly surprises inside. In this geology science project, you will see if the color and texture of the outside of a geode helps you tell, or predict, what the colors or crystals look like on the inside. Do you think that bumpy geodes have crystals inside that are bigger or smaller than smooth geodes? Can the outside color give you a clue to the inside color, or is that always a surprise? Try this science project to find out.
Terms and Concepts
- Bar chart
- How are geodes different from ordinary rocks?
- Why are crystals sparkly?
- In what kinds of rocks are geodes formed?
- Where do geodes form in these rocks?
- What shape are geodes?
- How big are geodes?
- Where can you find geodes on Earth?
This source provides an introduction to geodes:
- Moore, C. (2004). A Lesson About Geodes. Retrieved February 12, 2010, from http://www.magickeys.com/books/bitaba/geodes.html
This source will show you beautiful photos of geodes, as well as some theories about how they were formed:
- GMB Services. (2016). Welcome to RocksForKids. Retrieved October 25, 2016, from http://www.rocksforkids.com/
This source will explain how to break open a geode:
- The Geode Gallery. (2010). Retrieved February 12, 2010, from http://www.geodegallery.com/breakinggeodes.html
Materials and Equipment
Geodes (at least 10); available at science supply stores, such as Home Science Tools at
Try to get a variety of geodes that look different on the outside; for example, some smooth, some bumpy, as well as geodes that are different colors on the outside.
- Buying geodes from more than one source might help you get a better variety.
You might receive opened or unopened geodes. If unopened, you will also need:
- Impact protective goggles; available at science supply stores, such as Home Science Tools at www.homesciencetools.com
- Old sock
- Optional: Block of wood
- Optional: Chisel
- Try to get a variety of geodes that look different on the outside; for example, some smooth, some bumpy, as well as geodes that are different colors on the outside.
- Towel or paper towels
- Lab notebook
- Well-lit area
- Ruler, metric
- Optional: Magnifying glass
- Optional: Camera
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Preparing Your Geodes for Testing
If your geodes are unopened, then you will need to open them yourself. Read the instructions that come with your geodes about how to open them, or read these
- Note: Do not open your geodes yet. Just become familiar with what you will need to do to crack them open.
- Ask an adult to help you rinse any dirt off of your geodes (whether opened or unopened) in a sink or outside, so that you can see their color better.
- Dry the geodes well with a towel or paper towels.
- Set your geodes out on a table so that only their outsides are showing. If your geodes are unopened, you won't need to do anything special—just set them out on the table. If they are opened, though, you will need to turn them face down, as shown below.
Figure 3. This photo shows six geodes turned face down, so that only their outsides are visible.
Familiarize yourself with the different characteristics of the geodes.
- Write down all the different colors that you can see in your lab notebook.
- Close your eyes and run your fingers over the surface of the geodes. Are some smooth? Some bumpy? Write down the different types of surfaces in your lab notebook.
Preparing for Data Collection
- Make a data table in your lab notebook, like the one below:
|Geode Number||Description of Geode's Outside||Description of Geode's Inside|
|1|| || |
|2|| || |
|3|| || |
|4|| || |
|5|| || |
|6|| || |
|7|| || |
|8|| || |
|9|| || |
|10|| || |
Testing Your Geodes
Place one geode on a table in a well-lit area so that you can look at and feel its outside.
- In your lab notebook, describe the colors you see on the outside of Geode 1 in the space marked "Description of Geode's Outside."
- Close your eyes and feel the surface of the geode and write down, in this same space, how the surface feels to you.
- If your geode is already opened, turn it over so you can look at its inside and move on to step 4.
If your geodes are unopened, then you will need to crack them open, using the instructions that came with your geodes, or following these
- Remember to always wear eye protection when cracking geodes.
- Use great caution when using a hammer to avoid hitting anyone or smashing your fingers. Have a parent or teacher available to help with this step.
- You may find it easiest to open a geode by putting it in an old sock and hitting it with a hammer on a sidewalk, rather than using a chisel and hammer.
Place the cracked (opened) geode on the table in good light so that you can look at and feel its inside.
- In your lab notebook, describe the colors you see on the inside of Geode 1 in the space marked "Description of Geode's Inside."
Use a ruler to measure the size of one of the crystals, and write down the size in this same space.
- Are the crystals very tiny, like the size of 1 millimeter (the space between two closest lines on a metric ruler), or even smaller?
- Or, are the crystals larger, perhaps several millimeters across?
- Repeat steps 1–4 for all the remaining geodes.
Analyzing Your Data Table
Optional: Did you know scientists often show their data in a picture form called a graph? With a graph, you can often discover patterns in the data more easily than by looking at the numbers in the data table. If you've never made a graph before, you might want to ask a teacher, parent, or other adult for help. Try to make a graph of your data called a bar chart. You can plot the crystal size on the x-axis, and the number of rocks on the y-axis. You can make the graph by hand, or use a tool like Create A Graph. Steps to get started with Create A Graph are below.
- Go to Create A Graph and click on Bar since you will be making a bar chart.
- Click on the Data tab to enter your data.
- Don't worry about the graph title and labels just yet. Instead, focus on the data set. Change the data set "items" to 2, since you have two types of crystal sizes-small (less than 1 mm) and large (greater than 1 mm). Then change the data set "groups" to 2, since you have two groups of rocks-those with a smooth texture on the outside and those with bumpy texture on the outside.
- Under Group Label, enter the words Smooth Outside for Group 1 and Bumpy Outside for Group 2.
- Under Item Label, enter the words Small Crystals for Item 1 and Large Crystals for Item 2.
- Under Value for Group 1, enter how many rocks with smooth outsides had small crystals. Then right below it, enter how many rocks with bumpy outsides had small crystals.
- Under Value for Group 2, enter how many rocks with bumpy outsides had small crystals. Then right below it, enter how many rocks with bumpy outsides had large crystals.
- Click on the Preview tab to see your graph (bar chart).
- You can go back and add an x-axis label (Crystal Size), and a y-axis label (Number of Rocks) under the Data tab and click on Preview again to see the graph with labels.
- Once your graph looks the way you like, you can print it out.
- Looking at your data table or bar chart, what is the size of the crystals on the inside for most of the rocks with smooth outsides?
- Looking at your data table or bar chart, what is the size of the crystals on the inside for most of the rocks with bumpy outsides?
- Do you think you can tell (predict) what the size of the inside crystals are going to be by feeling the outside?
- Do the colors on the outside of your geodes help you tell what the color on the inside is going to be? Or, does the outside color not tell you anything about the inside color?
- Ask your friends or family to participate in your science project. Show each person an example of a geode containing large crystals and then an example of a geode containing small crystals. Let them feel the geodes and look at them from all sides. Then turn all 10 or more of your geodes face down on a table, so that their insides are not visible. Ask each person to put the ones that he or she thinks contain small crystals on one side of the table, and the ones that contain large crystals on the other side of the table. How many does each person get right and wrong? Is a person able to tell the crystal size inside a geode just from feeling the outside?
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