Backyard Biology

Nature stories from my backyard and beyond

Nature Activities

Eggshell Geode part 1

Monday, February 1, 2010

Many years ago the Museum of Science in Boston had an exhibit on crystals. The most popular part of the exhibit was the geode section. Geodes are hollow rock formations that fill with water containing certain minerals. Over the centuries, the water evaporates and the minerals precipitate out as beautiful crystal formations.
In this exhibit, visitors could purchase an unopened geode – a rock ball anywhere from 1 inch to 8 inches in diameter. geodes imageThere was no telling what the crystals inside looked like – or if there even were any crystals inside – until the geode was cracked open. For those visitors that couldn’t wait to get their geode home to break it open themselves, the Museum had set up a “geode cracking station” where the visitors could have their geode cracked open for them to expose the treasure inside.
It didn’t really matter to most visitors what the inside of their geode looked like. It was the suspense of finally seeing what Mother Nature had been creating in secret for perhaps the last few million years.

Breaking open and exposing for the first time the crystals inside the geode was such a wonderful experience that I thought I’d like to try to recreate it. Of course, any process that takes millions of years just won’t work for my Nature Activities, so I decided I needed to find a way to create my own geode in a matter of days or weeks. I decided that an “eggshell geode” was just what I needed. The process will be pretty much the same as a real geode, except that my “hollow rock formation” will not be a hollow in a rock, but a hollowed-out egg. I will fill the egg with water that contains certain minerals. The water will evaporate and the minerals will precipitate out into beautiful crystals. Then, when all is ready, I will carefully crack open the egg to expose my geode.

This Nature Activity will take place over a period of two months. This month we will do a little research on growing crystals. Next month we will take what we have learned and apply it to making our eggshell geode.

Growing Crystals

Wikipedia states that a crystal is a solid material, whose constituent atoms, molecules or ions are arranged in an orderly repeating pattern extending in all three special dimensions.

What this means in simple language is that if you dissolve a chemical in a liquid, such as water - as the water evaporates and the chemicals left behind attach themselves to each other, they do so in an orderly pattern, creating a specific shape – their crystal shape. Some materials, like quartz, form beautiful crystals, while others don’t. Crystals also need the right conditions to grow. Factors such as pH, temperature, impurities and space will all affect the growth of the crystal. In this activity, we will experiment with growing crystals from different chemicals to see which ones will work best for our egg geode.
A search on the internet will give many recipes for growing different types of crystals. I have taken some of the easiest ones to grow to use for our experiment, but as you gain experience with crystal growing, don’t be afraid to try others.

An ice cube tray makes a handy crystal-growing chamber. You can grow different crystals next to each other and easily compare the differences. You can also make slight modifications in your crystal recipes and see what the effects are.crystal tray image

In this picture, cubes 1 - 5 are table salt crystals. Cubes 1 and 2 were grown on a sponge. Cubes 3 and 5 had red food coloring added. Cube 3 was filled to a greater depth than cube 5.
Cubes 6-8 are brown sugar crystals. And cubes 9 – 11 are Epsom salt crystals. The picture below is a close-up of the epsom salt crystals in cube 3.

epsom crystals







My next experiment was growing Alum crystals and Copper Sulfate crystals. In this picture, copper sulfate crystals are shown growing in the copper sulfate solution in the cube, and one of the crystals removed from the solution.copper sulfate crystals image

Crystal-growing Recipes

Salt Crystals

  • 1 cup of hot water
  • ¼ cup of table salt
  • 2 teaspoons of vinegar
  • food coloring
  1. Add the salt and vinegar to the hot water and stir until you dissolve as much salt as possible.
  2. Add 1 drop of food coloring.
  3. Place some of this mixture in your ice-cube tray.

Epsom Salt Crystals

  • Epsom salt
  • Hot water
  1. Add the Epsom salt to the hot water and stir. Keep adding the Epsom salt until no more dissolves and some of the Epsom salt crystals remain in the water.
  2. Place some of this mixture in your ice-cube tray.

Sugar Crystals
Follow the instructions for Epsom Salt crystals with either white sugar or brown sugar replacing the Epsom salt.

Alum Crystals
Follow the instructions for Epsom Salt crystals with alum replacing the Epsom salt. Alum can be found in the spice section of the supermarket.

Copper Sulfate Crystals
Follow the instructions for Epsom Salt crystals with Copper Sulfate replacing the Epsom salt. Copper Sulfate can be found in the hardware store as a root killer.
Caution: copper sulfate is toxic and should not be touched with your bare hands. Be sure to read the hazard label on the package and follow the handling instructions.

Now that I have a little experience growing crystals, I am ready to try making some egg geodes. I have decided to make some geodes using salt crystals and some using copper sulfate crystals. Check in next month to see how my egg geodes turned out.

Note: for more crystal recipes, check out Dr. Anne Marie Helmenstine’s web site at