Easy and Fun Crystal Geodes You Can Grow
Natural geodes are hollow rock formations that are filled with crystals. They occur in sedimentary and volcanic rocks. Though you may not be lucky enough to live or visit a place where you can find your own geodes, you can make crystal geodes by speeding up nature's geological processes in the comfort of your own kitchen. This lens describes how to grow crystals inside natural minerals to make your own geode. The photo is a crystal geode that I grew.
Grow a Crystal Geode
Like Nature, Except Faster
Making a crystal geode is just like basic crystal growing, except instead of growing the crystals in a jar you are going to grow them on a plaster of paris 'rock' or inside an eggshell.
About the 'Rock'
The plaster or eggshell are calcium compounds, similar to actual rocks. While eggshell is a convenient substrate for your geode, you can grow larger crystals on the plaster because you can embed seed crystals in the plaster mixture when you mix it up.
You can crack open an egg, rinse out the shell, and peel away the membrane. The eggshell can be used as a cup to grow crystals only inside your geode or you can set the eggshell inside a container of crystal growing solution to grow crystals on as well as in the eggshell.
The other alternative is to mix just enough water in with some plaster of paris so that you can mold it into a geode shape. One way to do this is to press the wet plaster into the depression of an egg carton. If you have seed crystals or powder of the chemical you are going to use for the crystals, you can add a bit of it to the plaster to give your crystals a place to attach. Allow the plaster to set up before removing the geode from the mold. After the 'rock' is dry you can either pour crystal solution into it or else (like the eggshell) set the entire rock into crystal solution.
Growing Crystals
The easiest crystal to grow in your geode is alum. Make a saturated alum solution by dissolving as much alum as you can in a cup of hot water. You can add food coloring, if desired. The coloring will not color the alum crystals, but it will soak into the eggshell or plaster, making the crystals appear colored. Other crystal growing chemicals produce crystals that are truly colored.
Pour the alum solution into your geode. Don't allow the undissolved solid material to get into the geode since it will cause you to get smaller crystals. Allow your geode to sit undisturbed. Check on the status of your crystals the next day. Continue to allow your crystals to grow until you are satisfied with the appearance of your geode. You can dump any remaining solution down the sink. Allow your geode to air dry.
About the 'Rock'
The plaster or eggshell are calcium compounds, similar to actual rocks. While eggshell is a convenient substrate for your geode, you can grow larger crystals on the plaster because you can embed seed crystals in the plaster mixture when you mix it up.
You can crack open an egg, rinse out the shell, and peel away the membrane. The eggshell can be used as a cup to grow crystals only inside your geode or you can set the eggshell inside a container of crystal growing solution to grow crystals on as well as in the eggshell.
The other alternative is to mix just enough water in with some plaster of paris so that you can mold it into a geode shape. One way to do this is to press the wet plaster into the depression of an egg carton. If you have seed crystals or powder of the chemical you are going to use for the crystals, you can add a bit of it to the plaster to give your crystals a place to attach. Allow the plaster to set up before removing the geode from the mold. After the 'rock' is dry you can either pour crystal solution into it or else (like the eggshell) set the entire rock into crystal solution.
Growing Crystals
The easiest crystal to grow in your geode is alum. Make a saturated alum solution by dissolving as much alum as you can in a cup of hot water. You can add food coloring, if desired. The coloring will not color the alum crystals, but it will soak into the eggshell or plaster, making the crystals appear colored. Other crystal growing chemicals produce crystals that are truly colored.
Pour the alum solution into your geode. Don't allow the undissolved solid material to get into the geode since it will cause you to get smaller crystals. Allow your geode to sit undisturbed. Check on the status of your crystals the next day. Continue to allow your crystals to grow until you are satisfied with the appearance of your geode. You can dump any remaining solution down the sink. Allow your geode to air dry.
Amethyst Geode (Nasir Khan)
Natural Geodes
The exterior of most natural geodes consists of limestone (similar to the plaster of the man-made geode), with an interior of quartz or chalcedony. Sometimes the exterior is also quartz or a related stone, such as the amethyst shown in the photo.
Geodes at Amazon
Break Your Own Geodes
Amazon Price: $12.50 (as of 07/14/2009) 
Agate Geode OCCOs 1 Kilo (2.2 lbs) Assorted 5 to 9 Pieces - Grade B
Amazon Price: (as of 07/14/2009)
Break Your Own Geodes
Amazon Price: (as of 07/14/2009)
Crystal Geode Videos
Learn More about Geodes and Crystals
This is a collection of links to further reading about geodes as well as recipes for crystals that you can try with your homemade geode.
- Copper Sulfate Geode
- Alum crystals are probably the easiest crystals to grow in your geode, but copper sulfate crystals are interesting because they are naturally bright blue.
- Glow in the Dark Geode
- Another variation is to make a crystal geode that glows in the dark!
- Ammonium Phosphate Crystals
- This is the chemical found in most crystal-growing kits. It grows easily and can be tinted with food coloring.
- Geodes
- This is Wikipedia's entry for geodes, which contains concise information about what geodes are and how they form.
- Crystal Science Fair Projects
- Get ideas for ways to use crystals as science fair projects. Can you think of ways to use crystal geodes as a project?
- How to Grow Crystals
- Need a little extra help with the crystal-growing part of this project? This Squidoo lens contains more details about growing crystals.
- Top Crystal Growing Projects
- Here are some more crystal growing recipes for you to try with your geode.
- Crystal Photo Gallery
- Pretty, pretty crystal pictures!
- Crystal Geodes Review - Review of Smithsonian Crystal Growing Geodes Kit
- You can grow 'frosty diamonds' and 'purple amethyst geodes' with the Smithsonian Crystal Growing Geodes kit. Here's my review of this popular crystal growing kit.
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