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Impurity Ions and Gemstone Colors
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, it loses electrons and becomes an ion. In the case of chromium in aluminum oxide, it loses three electrons, so we say it’s trivalent. It has a valence of +3 (Cr3+).
The trivalent chromium impurity substitutes for the trivalent aluminum of corundum. The addition of the trivalent chromium colors a corundum gem pink or…
Dr. John Emmett
Dr. John Emmett is one of the world's foremost authorities on the heat treatment, physics, and chemistry of corundum. He is a former associate director of Lawrence Livermore National Laboratory and a co-founder of Crystal Chemistry, which is involved with heat treatment of gemstones.
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