Glow-in-the-Dark Geode

    Kids, how can you make a geode glow in the dark? It's very easy in this experiment. The 'rock' is a natural mineral (in this case an eggshell). You can use one of several common household chemicals to grow the crystals. And the glow comes from paint that you can get from a craft store.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    To Prepare the “Rock”:

    1. There are two ways to crack eggs. Carefully crack the top of the egg by tapping it on a countertop to make a deep geode with a smaller opening. Alternatively, crack the equator of the egg or have an adult partner carefully cut it with a knife. This will make a geode you can open and put back together.
    2. Discard the egg (or save for scrambled eggs).
    3. Rinse out the inside of the eggshell with water. Peel away the interior membrane to leave only the shell.
    4. Allow the egg to air dry or carefully blot it dry with a paper towel or napkin.
    5. Use a paintbrush or swab to coat the inside of the eggshell with glow-in-the-dark paint (such as GlowAway™ washable glowing paint).
    6. Set the painted egg aside while making the crystal-growing solution.

    To Make the Crystal Solution:

    1. Have an adult partner pour hot water (such as from a coffeemaker) into a cup.
    2. Stir borax or another crystal salt (alum, epsom salts, sugar, or table salt) into the water until it stops dissolving and you see some solid at the bottom of the cup.
    3. Add food coloring, if desired. Food coloring does not get incorporated into all crystals (e.g., borax crystals will be clear), but it will stain the eggshell behind the crystals, giving the geode some color. Neon green coloring looks great.

    To Grow the Crystals:

    1. Support the shell so that it won't tip over (for example, a nest can be made with a crumpled napkin set inside a cereal bowl).
    2. Pour the crystal solution into the shell so that it is as full as possible. Don't pour the undissolved solid into the eggshell, just the saturated liquid.
    3. Set the shell somewhere where it won't get knocked over. Allow crystals to grow for several hours (overnight is better) or as long as you like.
    4. When you are satisfied with the crystal growth, pour out the solution and allow the geode to dry.
    5. Phosphorescent paint is activated by exposing it to bright light; black light (ultraviolet) also produces a very bright glow. The duration of the glow depends on the paint (seconds to minutes).

    Note: Take appropriate safety cautions when handling the crystal solutions.

    "Geode" in room light  "Geode" under ultraviolet (black) light 


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    February 2008



    Dr. Anne Marie Helmenstine at