Waco Gem and Mineral Club, Waco, Tx

1.6 Growing Crystals and Making Geodes


1.6: Growing Crystals and Making Geodes

Growing Crystals

Some minerals grow into crystals in water solutions. This process can be observed using readily available materials, such as sugar, salt, alum, and Epsom salts dissolved into a saturated solution in boiling hot water. A saturated solution contains the maximum amount of salt that will dissolve in a given amount of hot water.


Crystal-building material: table salt, Epsom salts (from a pharmacy), or powder alum (from the grocery store spice section). Other materials you might use include borax, photographic fixer, sugar, or saltpeter. Copper sulfate (from pool supply or hardware stores) is used to create blue chalcanthite crystals. With young kids, though, it‘s probably best to stick with basics, such as table salt.


Measuring cups


Cooking pan

Glass jars


Stick or pencil

String (cotton twine), cut into small lengths, with a paperclip tied to one end

(optional) food coloring


Heat water to a boil, then turn off the heat

If using table salt, mix one-half cup of salt into three-quarters cup of hot water. If using Epsom salts, mix one-half cup Epsom salts into one cup of water. If using alum, mix one-quarter cup alum into one cup of water

Stir your solution. If all of your mineral dissolves, the solution is not yet saturated, and you should add a bit more mineral until no more will dissolve. (Note: you can make colorful crystals by adding a couple drops of food coloring.)

Place a few pebbles in the bottom of a glass jar and pour your solution over the pebbles. Or, tie string to a stick or pencil, pour your solution into a glass jar, and dip the string into the solution with a paperclip to weigh the string down, and leave it hanging there from the pencil

Set your jar aside in a spot where it won‘t be disturbed and don‘t bump or bounce it. Check every so often the next few days. As water evaporates, you‘ll see crystals forming on your pebbles or string

Assign different salts to different kids, and at your next monthly meeting, have everyone bring their jars to compare the different forms of crystals each produced.

In addition to home-made crystals, you can grow crystals using commercial crystal-growing kits. Check places like Ward‘s Natural Science (order their Earth Science and Geology catalogs; phone 1-800-962-2660).

Another source is Edmund‘s Scientific - 1-800-728-6999. Or check in toy stores, nature stores, or stores specializing in teacher supplies.

Making Geodes

Geodes are round or elliptical rocks with an outer shell and a hollow interior lined with crystals, often quartz or calcite. With this fun crystal-growing activity, kids can make their own geodes using eggshells or walnut shells.

If using eggshells, carefully crack eggs and pick out the best halves, choosing ones that don‘t have any long cracks in them. Carefully peel away the skin on the insides of the shells with tweezers or by rubbing with your fingers. If using walnut shells, crack a number of walnuts in half, clean out the meat, and choose shells lacking any long cracks.

Next, set your eggshell or walnut shell halves in egg cartons, where they‘ll be stable and secure. Using the same recipes described earlier for growing crystals, prepare a crystal-growing solution and pour or spoon a little into each egg- or walnut shell. Set the egg carton aside in a warm, dry spot where it won‘t be disturbed for the next week or more as the water evaporates, leaving you with sparkling geode halves.

The Women in Mining (WIM) Educational Foundation also provides a neat geode activity wherein you split a coconut, clean out the meat, drill a hole in one half, and lacquer the outside of both halves. Once all has dried, glue the two halves back together with silicone and pour a crystal-growing solution into the hole. A week or two later, pour out any remaining solution and cut open the coconut along the seam to reveal a sparkling interior.

 Note: Kids can use this activity toward satisfying requirement for the Earth Processes badge simultaneously (Activity 10.4.a. – A).