Step 2: Advanced Gemology

Lesson 4

I’ve faced many questions about the element chromium (Cr), the red coloration of minerals, and the gemstone red beryl. One of the many causes of color in gems and minerals is impurity ions. They are completely dissolved in the mineral and contribute to gemstone color.

The resulting color from an impurity in a mineral depends on the following factors.

  • The type of impurity ion.
  • The valence state of the impurity ion.
  • The concentration of the impurity ion.
  • The strength and symmetry of the crystal field on the impurity ion at the site at which it resides in the crystal. (This is determined by mineral type).

To explain this, I’ll use sapphire and ruby as my first gemstone examples. Chromium in corundum causes pink or red coloration.

Impurity Ions and Ruby and Sapphire Color

Just aluminum (Al) and oxygen (O) comprise pure corundum. The chemical formula, Al2O3, gives their relative amounts. Pure corundum doesn’t absorb any light from the far ultraviolet to the mid-infrared range. Thus, it’s absolutely colorless.

Now, chromium is a metal. However, when a chromium atom takes part in making a chemical compound,…