New Mexico Mineral Symposium — Abstracts

The colors that one sees when looking at a mineral or gemstone are due to the response of that person's eye to the energies of the light, the emission spectrum of the illumination, and, most importantly, physical phenomena in the material that cause some colors to be absorbed while others are undisturbed or enhanced. It is beyond the scope of this talk to do more than touch on the physiology of the eye that allows us to see colors. Likewise, we will not dwell on the emission spectra of various light sources. Rather, we will concentrate on the various ways in which materials, especially minerals and their heights of perfection, gemstones, produce color from white light.

Light is a form of energy (electromagnetic energy) and white light is a mixture of all of the visible energies (or wavelengths). In order for a mineral to cause color from white light it has to somehow perturb the balance of the light energies. Kurt Nassau2'3 has separated the causes of color into 15 mechanisms based on five physical groupings. Although there are some color mechanisms that depend on direct emission of certain colors, most of the mechanisms we are interested in depend on the ability of minerals to preferentially absorb certain energies of light. When these energies are removed from the white light the mineral is colored by the complimentary color as demonstrated by the CIE* Chromaticity Diagram.

Light absorption by the electrons of transition-metal or rare-earth-element (REE) atoms, either as major parts of the mineral chemistry or impurities, is one of the most important and well-known of the coloring mechanisms2'3'5. Most common rock-forming elements have electronic structures that mitigate against causing colors. On the other hand, transition metals and REEs have electrons that can be excited to open, higher-energy levels. The electrons gain the necessary energy for the excitation by absorbing a particular energy (color) from white light, and thus cause the mineral to show the complementary color. Three prime examples of this mechanism are rubies, emeralds, and alexandrites, but there are many, many more.

Another important and common coloring mechanism is intervalence charge transfer. This occurs when a valence electron from one atom transfers to the structure of a nearby atom (often of a completely different element), again by absorbing just the amount of energy needed to make the transfer. Examples include sapphire, lapis lazuli, and amazonite.

Part 2 of this talk (next year) will discuss two more absorption-type mechanisms—color centers (fluorite, smoky quartz, amethyst) and band-gap colors (yellow and blue diamonds, cuprite, cinnabar). Then it will describe how colors are caused by physical phenomena such as scattering (cat's eyes, stars, opalescence), dispersion (fire in diamonds), interference (labradorite), and diffraction (play of colors in opal).

*Commission Internationale de l'Eclairage

ACKNOWLEDGMENT, REFERENCES, AND SUGGESTED READING
This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.
 

References:

1. Fritsch, Emmanuel et al., Gem-quality cuprian-elbaite tourmalines from Sao Jose da Batalha, Paraiba, Brazil: Gems and Gemology, v. 26, no. 3, pp. 189-205.
2. Fritsch, Emmanuel, and Rossman, G. R. 1988, An update on color in gems, part 3: colors caused by band gaps and physical phenomena: Gems and Gemology, v. 24, no. 2, pp. 81-102. (Contains a table describing the causes of color for most gemstones.)
3. Fritsch, Emmanuel, and Rossman, G. R., 1987, An update on color in gems, part 1: introduction and colors caused by dispersed metal ions: Gems and Gemology, v. 23, no. 3, pp. 126-139.
4. Fritsch, Emmanuel, and Rossman, G. R., 1988, An update on color in gems, part 2: colors involving multiple atoms and color centers: Gems and Gemology, v. 24, no. 1, pp. 3-15.
5. Gems and Gemology contains numerous articles that include discussions of particular coloring mechanisms (such as ref. 9), too many to be enumerated here.
6. Nassau, Kurt, 1980, The causes of color: Scientific American, v. 243, no. 4, pp. 124-154. (Provides an excellent summary of the subject.)
7. Nassau, Kurt, 1981, Cubic zirconia: an update: Gems and Gemology, v 17, no. 1, pp. 9-19.
8. Nassau, Kurt, 1983, The physics and chemistry of color: the fifteen causes of color: John Wiley & Sons, New York, 454 pp.
9. Nassau, Kurt, and Valente, G. K., 1987, The seven types of yellow sapphire and their stability to light: Gems and Gemology, v. 23, no. 4, pp. 222-231.