How To Grow Purple Chromium Alum Crystals

Crystals That Resemble Amethyst Gems

Alum crystals
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Learn how to grow deep purple or lavender cubic crystals of potassium chromium sulfate dodecahydrate. In addition, you can grow clear crystals around the purple crystals, yielding a sparkling crystal with a purple core. The same technique can be applied to other crystal systems.

What You Need:

  • potassium chromium sulfate (chromium alum)
  • alum
  • water
  • clear glass jar
  • string
  • coffee filter or paper towel
  • pencil, knife, or stick (to suspend string)
  • spoon or stirring rod

Time Required: days to months depending on the desired size.

Here's How:

  1. The growing solution will consist of a chromium alum solution mixed with an ordinary alum solution. Make a chromium alum solution by mixing 60 g of potassium chromium sulfate in 100 ml water (or 600 g chromium alum per liter of water).
  2. In a separate container, prepare a saturated solution of ordinary alum by stirring alum into warm water until it will no longer dissolve.
  3. Mix the two solutions in any proportion that you like. The more deeply colored solutions will produce darker crystals, but it will also be harder to monitor crystal growth.
  4. Grow a seed crystal using this solution, then tie it to a string and suspend the crystal in the remaining mixture.
  5. Loosely cover the container with a coffee filter or paper towel. At room temperature (~25°C), the crystal can be grown via slow evaporation for as little time as a few days or as long as a few months.
  6. To grow a clear crystal over a colored core of this or any other colored alum, simply remove the crystal from the growing solution, allow it to dry, and then re-immerse it in a saturated solution of ordinary alum. Continue growth for as long as desired.


  1. A saturated solution of pure chrome alum will grow darker crystals, but the solution will be too dark to see through. Feel free to increase the concentration of chrome alum, but be aware that the solution becomes deeply colored.
  2. Notice that the chrome alum solution is a dark blue-green, but the crystals are purple!
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Your Citation
Helmenstine, Anne Marie, Ph.D. "How To Grow Purple Chromium Alum Crystals." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). How To Grow Purple Chromium Alum Crystals. Retrieved from Helmenstine, Anne Marie, Ph.D. "How To Grow Purple Chromium Alum Crystals." ThoughtCo. (accessed June 10, 2023).