Grow Alum Crystals That Resemble Simulated Diamonds

Alum Crystals That Look Like Diamonds

Alum crystals grow overnight into beautiful diamond-like jewels.
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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, grow a big alum crystal that looks a bit like a diamond. It only takes about an hour to grow small alum crystals, but getting a big crystals takes days to weeks.

Grow a Big Alum Crystal

  • Alum crystals are colorless, non-toxic crystals that are easy to grow.
  • Large crystals somewhat resemble diamonds, although they are much softer than the gemstones.
  • Expect growing a large crystal to take days or a couple of weeks.

What You Need for Alum Crystals

All you need to grow alum crystals are alum, hot water, and a container. Choose a clear container so you can watch the crystals grow. While not strictly necessary, it helps having a way to tie and suspend a crystal in the liquid. This helps it keep an ideal shape. A coffee filter or paper towel keeps dust out of your project, while still allowing good air circulation.

  • 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

There are actually a few different kinds of alum. The edible one in the grocery store is potassium alum. It grows clear crystals. Other types of alum include sodium, ammonium, selenium, and chrome alum. Chrome alum grows deep purple crystals. If you have access to the other chemicals, feel free to combine them to see what colors you get. But, check the labels for safety information. Some types of alum are non-toxic, but others are irritants and not edible.

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.

Solving Common Problems

The most common problem people experience growing alum crystals is that crystals not growing. If you don't see crystal growth within a day or two, there isn't enough alum in the liquid. Gently heat the liquid over a stove or in the microwave and try adding more alum powder. Crystals only grow if the solution is saturated. This is the point where no more solid dissolves.

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.
  3. Don't worry if you don't want to bother with the string! Crystals grow just fine on the bottom of the container. Use a spoon to scrape crystals away from each other so they won't grow together. The shape of crystals growing on a flat surface differs from the shapes that form when crystals are suspended.
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Your Citation
Helmenstine, Anne Marie, Ph.D. "Grow Alum Crystals That Resemble Simulated Diamonds." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). Grow Alum Crystals That Resemble Simulated Diamonds. Retrieved from Helmenstine, Anne Marie, Ph.D. "Grow Alum Crystals That Resemble Simulated Diamonds." ThoughtCo. (accessed June 2, 2023).