Impurity Ions and Mineral 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 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 red. We call it pink sapphire if it's pink. We call it ruby if it's red. Rubies contain more of the chromium impurity (roughly 0.5%) than pink sapphires (roughly 0.05%).

Trivalent Chromium and Mineral Colors

Trivalent chromium also colors emerald green and alexandrite both red and green. The strength of the crystal field at the site of the trivalent chromium impurity makes the difference in these gems' colors. Ruby, aluminum oxide, is a strong field material. Thus, the major chromium absorption bands end up in positions that absorb much of the blue-green and yellow light, giving the stone a red appearance.

Emerald, a beryl, is a weak field crystal. Thus, the chromium absorption bands shift towards the red, so more green light is transmitted through the crystal.

Between these examples lies alexandrite. It can show both colors, depending on the relative green/red color balance of the light source. (This is alexandrite's famous color change effect: green in daylight but red in incandescent light). Furthermore, alexandrite's crystal field is also quite anisotropic. The strength of the crystal field varies with direction in the crystal, resulting in a red-green pleochroism. (Keep in mind that color change and pleochroism are distinct effects).

More Coloration Factors

Remember that the perceived color of minerals depends on more than impurity ions. The absorption characteristics of the material, the spectral distribution of the light source, and the spectral response of the sensor (in this case, the eye), all play a role. A change in any of these characteristics may change perceived color. For example, different absorption spectra in rubies and emeralds alter perceived colors. A change in lighting from daylight to incandescent changes an alexandrite's perceived color. Finally, a change in sensor response, such as color blindness or green sunglass lenses, affects perceived color.

Manganese Impurity Ions and Red Beryl

Red beryl is sometimes quite inappropriately called "red emerald." (See our article regarding "white aquamarines" for a similarly misleading name). However, red beryl's coloration comes not from chromium but from trivalent manganese (Mn). This same impurity causes the pink to red coloration of tourmaline and the pink of kunzite, the pink variety of spodumene. However, trivalent chromium colors true hiddenite, the rare emerald-green spodumene variety. Often, kunzite comes out of the mine with green coloration. Is this caused by chromium? No. In fact, tetravalent manganese causes this coloration, which fades when exposed to light.

Tetravalent manganese comes from trivalent manganese exposed to ionizing radiation. Radioactive elements in the rocks produce this radiation naturally. Why does tetravalent manganese color spodumene nearly the same as trivalent chromium? Because when you strip four electrons off manganese, it looks almost the same as chromium with three electrons stripped off. In other words, Cr³⁺ and Mn⁴⁺ are isoelectronic.

Now that you understand all this, can you determine why sapphire with 5% chromium is a very deep red, while sapphire with 15% chromium is a deep green color?

Sources

Fritsch E., Rossman G.R. (1987). "An update on color in gems. Part 1: Introduction and colors caused by dispersed metal ions." Gems & Gemology, Vol. 23, No. 3, pp. 126-139.

Fritsch E., Rossman G.R. (1988). "An update on color in gems. Part 2: Colors involving multiple atoms and color centers." Gems & Gemology, Vol 24, No. 1, pp. 3-14.

Nassau K. (1983). The Physics and Chemistry of Color. John Wiley & Sons, New York.

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.

View All Articles