Causes of Color in Minerals and GemstonesPaul F. Hlava, Sandia National Laboratories

pfhlava@sandia.gov

The colors that one sees when looking at a mineral orgemstone are due to the response of that person's eye to theenergies of the light, the emission spectrum of the illumination,and, most importantly, physical phenomena in the material thatcause some colors to be absorbed while others are undis-turbed or enhanced. It is beyond the scope of this article to domore than touch on the physiology of the eye that allows us tosee colors. Likewise, we will not dwell on the emission spectraof various light sources. Rather, we will concentrate on thevarious ways in which materials, especially minerals and theirheights of perfection, gemstones, produce color from whitelight.

Light is a form of energy (electromagnetic energy) andwhite light is a mixture of all of the visible energies (or wave-lengths). In order for a mineral to cause color from white light ithas to somehow perturb the balance of the light energies. KurtNassau2'3 has separated the causes of color into 15 mecha-nisms based on 5 physical groupings. While there are somecolor mechanisms that depend on direct emission of certaincolors, most of the mechanisms we are interested in dependon the ability of minerals to preferentially absorb certain ener-gies of light. When these energies are removed from the whitelight the mineral is colored by the complimentary color as dem-onstrated by the CIE* Chromaticity Diagram.

Light absorption by the electrons of transition metal (orrare earth element - REE) atoms, either as major portions ofthe mineral chemistry or impurities, is one of the most impor-tant and well known of the coloring mechanisms2'3-5. Mostcommon, rock-forming elements have eiectronic structureswhich mitigate against causing colors. On the other hand,transition metals (and REE's) have electrons which can be ex-cited to open, higher energy levels. The electrons gain thenecessary energy for the excitation by absorbing a particularenergy (color) from white light and thus cause the mineral toshow the complimentary color. Three prime examples of thismechanism are rubies, emeralds, and alexandrites but thereare many, many more.

The second most important and common coloring mecha-nism is intervalence charge transfer. This occurs when a va-lence electron from one atom transfers to the structure of aclose-by atom (often of a completely different element) againby absorbing just the amount of energy needed to make the

transfer. Examples include sapphire, lapis lazuli, and amazonite.Color centers (also known as F-centers orfarbe {German for

color} centers) are created when atoms are oxidized or removed.This is usually done by radiation. In most cases the hole left be-hind is occupied by an electron trying to proxy for the missingatom. This electron comes from a neighboring atom and the un-paired electron left behind is prone to absorb light energy andthereby create colors. The most familiar examples of mineralscolored by color centers are amethyst and smoky quartz but fluo-rite, green diamonds, and brown topaz are also good examples.Color centers are one of the few coloring mechanisms that can beremoved by heating or exposing the mineral to strong light.

Band gap colors are produced in insulating and semiconduct-ing materials. They require an energy gap between the valenceand conduction energy in the electronic structure of an atom. Ifthe energy band includes all wavelengths of light the material iswhite or clear and an insulator. If the band includes the energiesof part of the visible spectrum the material is a semiconductor andcolored. Some insulators can be band-gap colored by impurities.Examples of minerals colored by band gap mechanisms are dia-monds, cinnabar, and cuprite.

Scattering of light can cause colors to appear because bluelight is more prone to scattering than red. Scattering is caused bysubmicroscopic (the finer the better) grains of solid or liquid, mate-rial. It can even be caused by random collisions of gas moleculesin the atmosphere. Scattering is responsible for the blue of thesky, the white of clouds (and bull quartz), and the red color of sun-sets. Minerals that display a special case of scattering are moon-stones, cat's eyes, and asterated (star) stones.

Interference colors are caused when light travels obliquelythrough materials with thin layers of differing refractive index. Thelayers have to be about as thick as a wavelength of light. A coher-ent ray of light shining through the material is dispersed in the newmedium. At each interface some of the light is reflected back upand some continues on down. If the layers are of such a thicknessthat a particular color is retarded by exactly one or a few integralwavelengths, the reflected ray of color and its refracted then re-flected counterpart will constructively interfere with each other andthat color will be bright. Those colors that are an integer and onehalf retarded will be destructively interfered and therefore can-celled out. Retardations between those extremes are muted. Theeffect is the schiller we associate with oil slicks, labradorite, cryp-toperthite-type moonstones, etc.

Diffraction can be considered a special case of interferencecaused, not by lamellae, but by layers of fine spheres. Eachsphere scatters the light impinging on it in a radial fashion. As the

Figure 1: Illustrated are two examples of coloring mechanisms.The blue of the sapphire is caused by intervalence chargetransfer while the white star is caused by scattering.

Figure 2: The Hope diamond which illustrates band gap colorcaused by a (natural) dopant (boron).

Continued on page 16

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Causes of Color in Minerals and GemstonesContinued from page 14

layers are tilted, different wavelengths are constructively inter-fered with in different directions producing not one color but aplay of colors. The spheres have to be perfectly round, of thesame, exact size (about the size of light waves), and packedinto perfect order or the play of colors will be killed. The perfectexample is precious opal. An imperfect example is the rainbowof colors one see when viewing a bright source of light througha fine screen or the cloth of an umbrella. •

"Commission Internationale de I'Eclairage

Acknowledgements, References, and Suggested Reading1. This work was supported by the United States Departmentof Energy under Contract DE-AC04-94AL85000. Sandia is amultiprogram laboratory operated by Sandia Corporation, aLockheed Martin Company, for the United States Department ofEnergy.2. Kurt Nassau, 'The Physics and Chemistry of Color: The Fif-teen Causes of Color", John Wiley & Sons, New York, 1983,454 pp.3. Kurt Nassau, "The Causes of Color", Scientific American, vol.243, no. 4, Oct., 1980, pp. 124 - 154 (Provides an excellentsummary of the subject.)4. Kurt Nassau,, "Cubic Zirconia: An Update", Gems and Gem-ology, vol 17, no. 1, Spring 1981, pp. 9 -195. Emmanuel Fritsch and George R. Rossman, "An Update onColor in Gems, Part 1: Introduction and Colors Caused by Dis-persed Metal Ions", Gems and Gemology, vol 23, no. 3, Fall1987, pp. 126-1396. Kurt Nassau and G. Kay Valente, "The Seven Types of Yel-low Sapphire and Their Stability to Light", Gems and Gemology,vol 23, no. 4, Winter 1987, pp. 222 - 2317. Emmanuel Fritsch and George R. Rossman, "An Update onColor in Gems, Part 2: Colors Involving Multiple Atoms andColor Centers", Gems and Gemology, vol. 24, no. 1, Spring1988, pp. 3- 158. Emmanuel Fritsch and George R. Rossman, "An Update onColor in Gems, Part 3: Colors Caused by Band Gaps and Physi-cal Phenomena", Gems and Gemology, vol 24, no. 2, Summer1988, pp. 81 -102 - (Contains a table describing the causes ofcolor for most gemstones.)9. Emmanuel Fritsch et al., "Gem-Quality Cuprian-Elbaite Tour-malines from S§o Jose da Batalha, Paraiba, Brazil", Gems andGemology, vol 26, no. 3, Fall 1990, pp. 1139 - 205"Gems and Gemology" contains numerous articles that includediscussions of particular coloring mechanisms (such as ref. 9).Too many to be enumerated here.

r— — — — — — — — — — — — — — — — — i

An ApologyIn our October 2000 issue we published the article

"Scanning Scanning Electron Microscopy" by a fictitious au-thor. The object of this article, if not as a satire, was at least tobe humorous. In this article several individuals, including Tho-mas George, and their considerable accomplishments werementioned. Due to the nature of the article, it has come to ourattention that some may think that were making "light" ofthese accomplishments. I wish to advise that this was not ourintent and specifically to offer our apologizes to Dr. Georgeeand the others

.. . Don Grimes, EditorP R I N C E T O N S I B 1 A . T E C H

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