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 Nassau23 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 complementary color as demonstrated by the CIE* Chromaticity Diagram.

Last year's talk concentrated on the nature of light, a bit on the physiological aspects of color vision, a quick listing of the 15 mechanisms (according to Nassau), and discussions of two well-known coloring mechanisms—transition metal absorption and intervalence charge transfer. Transition metal absorption examples included ruby, emerald, and alexandrite. Intervalence charge transfer examples included sapphire, amazonite, and lapis lazuli.

This year's talk will briefly review some basic concepts of last year's talk and then go on to 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).

Color centers (also known as F-centers or farbe {German for color) centers) are created when atoms are oxidized or removed. This is usually done by radiation. In most cases the hole left behind is occupied by an electron trying to proxy for the missing atom. This electron comes from a neighboring atom, and the unpaired electron left behind is prone to absorb light energy and thereby create colors. The most familiar examples of minerals colored by color centers are amethyst and smoky quartz, but fluorite, green diamonds, and brown topaz are also good examples. Color centers are one of the few coloring mechanisms that can be removed by heating or exposing the mineral to strong light.

Band-gap colors are produced in insulating and semiconducting materials. They require an energy gap between the valence and conduction energy in the electronic structure of an atom. If the energy band includes all wavelengths of light the material is white or clear and an insulator. If the band includes the energies of part of the visible spectrum the material is a semiconductor and colored. Some insulators can be band-gap colored by impurities. Examples of minerals colored by band-gap mechanisms are diamonds, cinnabar, and cuprite.

Scattering of light can cause colors to appear because blue light is more apt to be scattered than red. Scattering is caused by submicroscopic (the finer the better) grains of solid or liquid material. It can even be caused by random collisions of gas molecules in the atmosphere. Scattering is responsible for the blue of the sky, the white of clouds (and bull quartz), and the red color of sunsets. Minerals that display a special case of scattering are moonstones, cat's eye, and asterated (star) stones.

When white light enters a transparent medium of differing refractive index at an oblique angle, the light path is bent; it is refracted. Each of the colors is refracted to a slightly different angle; the colors are dispersed. If the light is allowed to exit the medium via a plane parallel to the entrance surface, the light will recombine to white. But if the entrance and exit planes are not parallel, the various colors will exit via different paths and produce little rainbows of color. Dispersion colors are the "fire" that we associate with faceted stones, especially diamonds.

Interference colors are caused when light travels obliquely through materials with thin layers of differing refractive index. The layers have to be about as thick as a wavelength of light. A coherent ray of light shining through the material is dispersed in the new medium. At each interface some of the light is reflected back up and some continues on down. If the layers are of such a thickness that a particular color is retarded by exactly one or a few integral wavelengths, the reflected ray of color and its refracted then reflected counterpart will constructively interfere with each other and that color will be bright. Those colors that are an integer and one half retarded will be destructively interfered and therefore cancelled out. Retardations between those extremes are muted. The effect is the schiller we associate with oil slicks, labradorite, cryptoperthite types moonstones, etc.

Diffraction can be considered a special case of interference caused, not by lamellae, but by layers of fine spheres. Each sphere scatters the light impinging on it in a radial fashion. As the layers are tilted, different wavelengths are constructively interfered with in different directions producing not one color but a play of colors. The spheres have to be perfectly round, of the same, exact size (about the size of light waves), and packed into i perfect order or the play of colors will be killed. The perfect example is precious opal. An imperfect example is the rainbow of colors one sees when viewing a bright source of light through a fine screen or the cloth of an umbrella.

*Commission Internationale de l'Eclairage

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. "Gems and Gemology" contains numerous articles that include discussions of particular coloring mechanisms (such as ref. 9), too many to be enumerated here.
2. Fritsch, E., 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.
3. Fritsch, E., 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.
4. Fritsch, E., 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.)
5. Fritsch, E., et al., 1990, Gem-quality cuprian-elbaite tourmalines from Sdo Jose da Batalha, Paraiba, Brazil: Gems and Gemology, v. 26, no. 3, pp. 189-205.
6. Nassau, K., 1980, The causes of color: Scientific American, v. 243, no. 4, pp. 124-154. (Provides an excellent summary of the subject.)
7. Nassau, K., 1981, Cubic zirconia???an update: Gems and Gemology, v. 17, no. 1, pp. 9-19.
8. Nassau, K., 1983, The physics and chemistry of color???the fifteen causes of color: John Wiley & Sons, New York, 454 pp.
9. Nassau, K., 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.