Grow Your Own Simulated Diamonds With a Big Alum Crystal

Alum Crystals That Look Like Diamonds

Alum crystals grow overnight into beautiful diamond-like jewels.
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by Anne Marie Helmenstine, Ph.D.
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Updated February 13, 2018

Alum is found in the 'spices' section of the grocery store. That little jar contains small white crystals that, with a bit of time and effort, you can grow a big alum crystal that looks a bit like a diamond. This takes days to weeks.

What You Need for Alum Crystals

  • 1/2 cups hot tap water
  • 2-1/2 tablespoons alum
  • nylon fishing line
  • pencil, ruler, or knife
  • 2 clean jars
  • spoon
  • coffee filter/paper towel

Grow the Crystals

  1. Pour 1/2 cup of hot tap water into a clean jar.
  2. Slowly stir in alum, a little at a time, until it stops dissolving. Don't add the whole amount; just enough to saturate the water.
  3. Loosely cover the jar with a coffee filter or paper towel (to keep dust out) and allow the jar to sit undisturbed overnight.
  4. The next day, pour the alum solution from the first jar into the clean jar. You will see small alum crystals at the bottom of the jar. These are 'seed' crystals that you will use to grow a big crystal.
  5. Tie nylon fishing line around the largest, best-shaped crystal. Tie the other end to a flat object (e.g., popsicle stick, ruler, pencil, butter knife). You will hang the seed crystal by this flat object into the jar far enough so that it will be covered in liquid, but won't touch the bottom or sides of the jar. It may take a few tries to get the length just right.
  6. When you have the right string length, hang the seed crystal in the jar with the alum solution. Cover it with the coffee filter and grow a crystal!
  7. Grow your crystal until you are satisfied with its size. If you see crystals starting to grow on the sides or bottom of your jar, carefully remove your crystal, pour the liquid into the clean jar, and put the crystal in the new jar. Other crystals in the jar will compete with your crystal for alum, so it won't be able to get as big if you let these crystals grow.

Crystal Growing Tips

  1. You can use sewing thread or other string instead of nylon fishing line, but crystals will grow on the entire length of the submerged string. Crystals don't adhere to nylon, so if you use it, you can get bigger and better crystals.
  2. Alum is an ingredient used to make pickles. It makes them crispy.