Cave Crystals

Note: This activity takes several days for the crystals to form. See Preparations for more information.

Students will:

  • investigate deposition, the process that allows growth of different secondary cave formations,
  • recognize that different chemical compounds produce different speleothem types,
  • identify several speleothems.


  • 2 pieces of cotton yarn or string
  • 4 jars or cups of the same size
  • 2 saucers
  • Epsom salts
  • baking soda (sodium bicarbonate) or washing soda (sodium carbonate)
  • very warm water
  • spoon or coffee stirrer
  • several books and internet sites about caves and speleothems

On the first day that you start learning about cave set these two experiments up: Rock Candy Crystal and Forming Cave Popcorn will be set up. Observations will be made by the class each day on the progress of these experiments.

Dripping Crystals


1. Tell the students that they will be conducting an experiment that will demonstrate how water can deposit minerals to create cave formations, or "speleothems" (secondary mineral deposits formed in caves).

2. Ask the students if they have ever seen stalactites or stalagmites. Do they know which is which? Stalactites hang tight to the ceiling and stalagmites come up from the ground and might reach the ceiling someday. How do they form? Discuss how water can carry calcite in solution then deposit the calcite in crystalline form in a cave. If you would like use sugar, salt, Jell-O, or Kool-Aid as examples of minerals dissolving into solution.

3. Explain that the process would take millions of years in a cave. The students will speed up the process in the classroom by using a concentrated solution and sunlight (to aid evaporation).

4. Students may create the set-up, help with it, or watch as you complete the steps. If the students create the set up have them work in small groups.


  • Dissolve as much Epsom salt (or washing soda) as you can in two jars or cups half full of very warm water.
  • Soak a piece of string (yarn) in the solution. Lay the yarn on the jars so that both ends are well inside the solution in each jar and the middle is over the saucer with a dip in the middle. The closer the dip is to the saucer the great chance you have of creating a column. (Epsom salt creates better popcorn type crystals, washing soda is harder to find, but makes better stalactites.)
  • Put just a touch of dry Epsom salt or washing soda on the saucer. Leave the jars for several days.

5. The solution in the jars will drip onto the saucer, creating a stalactite and stalagmite, and eventually a "column" (when they grow together). Hopefully this will occur… it doesn't happen every time.

6. Set up the second set of jars at the same time, using baking soda instead of Epsom salt. Within 48 hours, the students will notice that the crystals look very different in comparison. The baking soda will form delicate "popcorn" formations along the string.

7. As the students check their experiment each day, discuss the differences between the baking soda and Epsom salt deposits. Why are they different?

10.Discuss what happens when carbonic acid degasses, as it drips or seeps out of a cave wall. Degassing is the process where the acid can no longer hold anything in solution and must deposit it. Why does this cause deposition of calcite?

11. Do you notice crystals in other places than at the dip in the string? Where and why?

12. What property of water allows the string to stay wet and drip water? (Osmosis)

13. Give the students time during class to look at the included reference materials. Have them familiarize themselves with the following formations: stalactite, stalagmite, soda straw, column, popcorn, frostwork, boxwork, and flowstone.

14.Watch the water level in the glasses and discuss formations (speleothems), water levels, groundwater, and recharge. Notice the water level goes down and does not get refilled. Much of the water is being pulled into the yarn. Would this happen in a cave situation? (Yes, the water would drip and go further into the ground.) If there is no recharge, what will eventually happen? What would this mean to the cave system?

Hint: Be careful with the Epsom salts. The crystals are very delicate. You will need to add quite a lot Epsom salts and make sure they are dissolved in solution. It will be several days before they crystallize on the string, but they should. If the formation gets heavy, it will break off. Watch the crystallization below the drip it can be rather interesting.

Prerequisite: A basic understanding of dissolved substances and evaporation.

Making Rock Candy


  • 1 cup water
  • 3 cups sugar
  • clean glass jar
  • cotton string
  • pencil or knife
  • food coloring (optional)
  • 1/2 tsp to 1 tsp flavoring oil or extract (optional)
  • Lifesaver candy (optional)
  • pan
  • stove or microwave


1. Pour the sugar and water into the pan.

2. Heat the mixture to a boil, stirring constantly. You want the sugar solution to hit boiling, but not get hotter or cook too long. If you overheat the sugar solution you'll make hard candy, which is nice, but not what we're going for here.

3. Stir the solution until all the sugar has dissolved. The liquid will be clear or straw-colored, without any sparkly sugar. If you can get even more sugar to dissolve, that's good, too.

4. If desired, you can add food coloring and flavoring to the solution. Mint, cinnamon, or lemon extract are good flavorings to try. The juice from a lemon, orange, or lime is a way to give the crystals natural flavor, but the acid and other sugars in the juice may slow the crystal formation.

5. While your solution cools, prepare your string. Use cotton string because it is rough and non-toxic. Tie the string to a pencil, knife, or other object that can rest across the top of the jar. You want the string to hang into the jar, but not touch the sides or bottom. You will want to weight your string so it hangs into the container so tie a Lifesaver to the bottom of the string. (Don't use anything toxic.)

6. You want to 'seed' the string with crystals so that the rock candy will form on the string rather than on the sides and bottom of the jar. To do this, dampen the string with a little of the syrup (dissolved sugar water) you just made then dip the string in sugar.

7. Once your solution has cooled, pour it into the clean jar. Suspend the seeded string in the liquid. Set the jar somewhere quiet. Cover the jar with a paper towel or coffee filter to keep the solution clean.

8. Check on your crystals, but don't disturb them.

9. You can help your crystals grow by removing (and eating) any sugar 'crust' that forms on top of the liquid. If you notice a lot of crystals forming on the sides and bottom of the container and not on your string, remove your string and set it aside. Pour the crystallized solution into a saucepan and boil/cool it (just like when you make the solution). Add it to a clean jar and suspend your growing rock candy crystals. You can watch a video about making rock candy if you would like to see what to expect at:

10. When you are satisfied with the size of your rock candy, remove it from the container and then (after examining the crystals) eat it! Ideally you want to allow the crystals to grow for 3-7 days

Demonstration of boxwork formation.

One of the formations of Wind Cave is boxwork and it was formed very differently from the rest of the cave formations.


Tell the students to use their imagination. Your hand will represent the limestone of the cave. When the limestone was deposited pockets of gypsum were tucked inside. (Put a small amount of soft modeling clay inside your hand so that no one can see) Gypsum is unique and when it gets wet it expands. (Push the modeling clay with your hands so you it pushes out through the cracks your fingers apart to hold it). The gypsum expanded and cracked the limestone, like your fingers are spread apart. Tell students that as the gypsum continued to expand it filled in all the cracks it had made in the limestone. Over time, the gypsum turned into something called calcite. Over more time the limestone, when the cave filled with water, the water eroded the limestone away, (refer to previous activity) and exposed the boxwork. (Remove your fingers from the soft modeling clay and hold it up for the class to see) We are left with the calcite fins we call boxwork today. For a picture of boxwork, visit the park's website at

Forming Crystals (continued from day 1)

Procedures: - Popcorn

1. Examine the string and the baking soda/Epsom salt experiment.

2. Explain to students that as the water evaporates from the string the baking soda is left behind and more water and baking soda is pulled from the jar. As more water evaporates more baking soda is left and the crystals grow larger.

3. Students will finish their observations of the growing speleothems and write a summary of their formation.

Small, knobby growths of calcite on the cave walls are called cave popcorn. Popcorn commonly forms in one of two ways in the cave: where water seeps uniformly out of the limestone wall and precipitates calcite; or, when water drips from the walls or ceilings of the cave and the water splashes on the floor or on ledges along the walls. This splashing action causes loss of carbon dioxide and the subsequent precipitation of calcite. Visit to see an example of cave popcorn.

Procedures: Rock candy and dog tooth spar

1. Take out the rock candy that the class started the previous week.

2. Explain to the students that the sugar which was dissolved inside the water formed the crystals as the water evaporated.

3. Have students make their last observation.

4. Pass out pieces of the candy to each student to have.

Dogtooth spar is a speleothem found in limestone caves. They are calcite crystals that resemble dogs' teeth (hence the name). In Wind Cave the sharply tooth-shaped crystals are often found in open space including veins, fractures, and geodes. For an example of dog tooth spar visit:

Last updated: April 10, 2015

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