AN UPDATE on COLOR in GEMS. PART 3: COLORS CAUSED by BAND GAPS and PHYSICAL PHENOMENA by Emmanuel Fritsch and George R

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AN UPDATE ON COLOR IN GEMS. PART 3: COLORS CAUSED BY BAND GAPS AND PHYSICAL PHENOMENA By Emmanuel Fritsch and George R. Rossman

The previous two articles in this series de- 11 of the colors discussed in parts 1 and 2 of this series scribed the origins of color in gems that A (Fritsch and Rossman, 1987 and 1988) arise from derive from isolated structures of atomic processes in which electrons are localized on a single atom dimensions-an atom (chromium in emer- or are delocalized over no more than a few atoms. The ald), a small molecule (the carbonate colors that arise from these processes depend on the group in Maxixe beryl), or particular presence of specific minor components or defects in the

groupings of atoms (Fez+-0-Fe:f + units in cordieiite). The final part of this series host crystal. However, colors can arise, though less com- is concerned with colors explained by monly, from processes that involve the entire crystal, band theoiy, such as canary yellow &a- through either its electronic structure (band theory) or its monds, orby physical optics, such as internal texture (physical phenomena such as interference play-of-color in opal. In the case of band effects, diffraction effects, scattering, and inclusions; see theory, the color-causing entity is the very figure 1).These, the most unusual causes of color in gems, structure of the entire crystal; in the case are covered in this last article of our series. of physical phenomena, it is of inicro- scopic dimension, but considerably larger BAND THEORY than the clusters of a few atoms previ- In contrast to the processes described in the first two parts ously discussed. of this series, the electrons in some gem minerals can be delocalized over the entire crystal, and produce color through their interaction with visible light. Such delocal- ization is a characteristic property of most metals and semiconductors. The physical theory that describes the ABOUT THE AUTHORS cause of color in such materials is called band theory. Dr. Fritsch is research scientist at the Gemologi- Examples of the various gem colors explained by this cal Institute 01 America, Santa Monica, Calilornia. theory are presented in table 1. Dr. Rossman is prolessor of mineralogy at the In numerous solid materials, billions of atoms contrib- California Institute of Technology, Pasadena, Cali- lornia. ute to the possible energy levels, which are so numerous and so close together that they are considered collectively Acknowledgments: E. F. wishes to thank Professor Georges Calas, Mr. Pierre Bariand, and Ms. Anne as an energy band. This is of particular interest in the case Voileau for their help and encouragement in writ- of some semiconducting and metallic minerals (Marfunin, ing the original French version of this article. Spe- 1979a). There arc two bands in these solids: a low-energy cial appreciation goes to Ms. Pat Gray lor typing the original English manuscript and improving the valence band that is fully populated with electrons, and a translation, and to Ms. Ruth Patchick for word pro- high-energy conduction band that is generally empty cessing the many revisions, especially the tables. (figure 2). The energy that separates these bands is well The authors are also grateful to Ms, Laurel Bart- let!, Dr. James Shigley, Mr. John Koivula, and defined and is called a "band gap." This energy separation is Dr. John Hummel tor their constructive comments. of dramatic importance to the optical properties of certain 0 1988 Gemological Institute of America types of gemstones.

Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 81 Figure 1. Colors in gem materials can be caused by a wide variety of processes. This article explains color-producing mechanisms related to band theory and physical phenomena. Examples of the latter include diffraction in opal (in the center top and bottom right), scattering of light in "moonstone" feldspar (the necklace), and coloration by inclusions in fire opal (center bottom) and "siinstone" feldspar (bottom left). The necklace is courtesy of Elise Misiorowski; photo by Robert Weldon.

For these gemstones, transitions between When the energy of the band gap is greater than bands rather than between energy levels of single the maximum energy of the visible range (i.e., the atoms are responsible for the color. These "inter- violet light), visible light does not supply enough band transitions" occur when electrons from the energy to cause an electron to jump from the lower valence band receive sufficient energy by absorb- band to the upper one (figure2A). Consequently, all ing light to "jump" over the band gap and reach the of the visible spectrum is transmitted (none is conduction band. As illustrated in figure 2, three absorbed) and, in the absence of impurities or different scenarios are possible for interband tran- defects, the mineral is colorless. Such materials - sitions. e.g., corundum, beryl, quartz, diamond, and topaz,

TABLE 1. Types of gem materials for which color can be explained by the band theory and examples of the colors produced,

Origin of color Type of material Color Examples

Band gap less than the Conductors and some Violet to blue Covellite (Berry and Vaughan, 1985) energy of visible light semiconductors = colored opaque Yellow Gold, pyrite (Nassau, 1975; materials with metallic luster Fritsch, 1985) Red Copper (Nassau, 1975) White Silver, platinum (Nassau, 1975) Band gap in the Some semiconductors Red Cuprite, cinnabar (Fritsch, 1985) visible range Band gap greater than the Some semiconductors and all Intrinsically Diamond, corundum, beryl, quartz, energy of visible light insulators colorless topaz, fluorite (Fritsch, 1985) Color modified by minor Some semiconductors Blue Type llb diamond, containing components dispersed boron atoms (Collins, 1982) Yellow Type Ib diamond, containing dispersed nitrogen atoms (Collins, 1982)

82 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 as well as many other oxides and silicates-are quence, the mineral usually appears black and inherently electrical insulators. opaque. All metals have just such a small band gap When the energy of the gap is less than the or no band gap at all. They appear, however, to be energy of violet light (i.e., is in the visible range), shiny (metallic luster) because their electrons the most energetic radiations in the visible range quickly return to their original energy level, emit- (violet to blue to green] are absorbed, leaving the ting the exact same energy (light) that they for- low-energy range unaffected, that is, transmitted merly absorbed (Nassau, 1975b).In some metals, (figure 2B). The exact energy of the band gap varies the number of available excited states may vary among different materials, so the transmitted throughout the conduction band, so that some color will also vary. Usually band-gap colors range wavelengths are absorbed and re-emitted more from deep yellow to deep red. Cuprite and cinnabar efficiently than others, thus producing color. Al- (figure 3) are colored red by such a process. though silver and platinum absorb and emit all The energy in the band gap may be even less wavelengths with about the same efficiency and than the lowest energy of the visible spectrum appear white, gold (or pyrite) absorbs and emits (red).In such a situation, all wavelengths of visible more yellow than the other wavelengths and so light will cause a transition from the valence band gets its distinct golden coloration (again, see fig- to the conduction band, so the whole visible ure 3). spectrum is absorbed (figure 2C). As a conse- The band gaps discussed thus far are an intrin-

. . Figure. 2..The three possible types of intrinsic coloration of gem materials are.explained by examining the width of the band gap in relation to the visible range. (A)Band gap greater than the energy of the visible range: All visible radiation is transmitted and the gem is intrinsically colorless. (B) Band gap in the visible range: Only the high-energy part of the spectrum (violet to blue to green) is absorbed, and the gem is yellow to red. (C) Band gap less than the energy of visible light: All visible radiation is absorbed and the material is black, or displays metallic colors due to re-emission. Artwork by Ian Newell.

Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 83 Figure 3. The red color of cuprite (the cushion cut) and cinnabar (the pentagon cut) has little to do with the fact that these gems contain copper and mercury, respectively. The color occurs because the band gap of these minerals is within the visible range: All wavelengths from violet to orange are absorbed, so that only red light is transmitted. The band gap in gold is much smaller than the energy of the visible range: All visible light is absorbed but some wavelengths are re- emitted preferentially, giving gold its yellow color and metallic luster. (Re- member that the energy scale is in- verse to the wavelength scale.) Photo by Robert Weldon. sic property of the material; they are ultimately nitrogen-related N3 color center, which produces directly related to its chemical composition and the familiar Cape series of absorption lines. atomic structure. I11 some semiconductors, how- Boron has one less electron than carbon, which ever, color is caused by small amounts of impurity follows it in the periodic table of elements. There- atoms that normally do not produce color in fore, boron is an electron "acceptor" when substi- intrinsically colorless minerals. Specifically, these tuting for carbon in diamond. It contributes its atoms can introduce electronic energy levels at an own electron energy band, which is situated energy between the valence band and the conduc- within the diamond band gap (figure 4C). The tion band of the host mineral (seefigure 4).Some of excitation of an electron from the diamond valence the most striking examples are canary yellow and band to the acceptor level requires only a very low fancy blue diamonds, which contain isolated nitro- energy, in the infrared range (Collins, 1982). Be- gen and boron atoms, respectively. Although pure cause the boron energy band is broad, it can cause (colorless)diamonds are insulators, they may also absorption extending from the infrared up to 500 be considered semiconductors with a band gap so large that they have neither color nor appreciable electrical conductivity (figure 4A). Nitrogen can Figure 4. The band gap in pure diamond is much easily substitute for carbon, which it follows in the greater than the visible range (A), so this gem is in- periodic table of elements. Because nitrogen pos- trinsically colorless. However, a substitution a1 ni- sesses one more electron than carbon, however, it trogen atom introduces a level that donates elec- becomes an electron "donor" when it substitutes trons to the diamond conduction band (B), creating for carbon in diamond. This additional electron an absorption in the ultraviolet that extends into contributes an additional energy level situated the blue end of the visible range (see spectrum); above the diamond valence band, but below the such stones are of an intense yellow color, and are diamond conduction band (figure 4B). However, therefore called "canary" (type Ib) diamonds. By because this donor level has a finite width, light of contrast, a boron atom substituting for carbon can a variety of wavelengths extending from the ultra- introduce a broad energy level available for elec- violet into the visible range up to 560 nm (green) trons from the diamond valence band (C), which will induce an absorption of the near-infrared and will be absorbed, creating a strong yellow color. the red end of the visible range (see spectrum), giv- This type of coloration occurs only in type Ib ing a blue hue to such a stone (type llb diamond). diamonds, in which isolated nitrogen atoms sub- These three mechanisms are illustrated by the col- stitute for carbon atoms in the proportion of about orless, De Beers synthetic type Ib yellow (photo 1 to 100,000 (Collins, 1982).This color is distinct 0 Tino Hammid), and natural dark blue diamonds from the yellow color commonly caused by the shown here. Artwork by Jan Newell.

84 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 A COLORLESS B TYPE 1-B C TYPE 11-0 DIAMOND DIAMOND DIAMOND (Yellow) (Blue) 'I--- ^

0 - VISIBLE RANGE WAVELENGTH (nm) . I t DIAMOND CONDUCTION BAND

DIAMOND VALENCE BAND

Color in Gems, Part 3 GEMS &. GEMOLOGY Summer 1988 85 Figure 5. Minute amounts of boron con- tribute the intense blue color to blue diamonds. Yellow in diamonds can arise from a variety of processes, 011 of which are related to the presence of nitrogen impurities. The possible origins of color for orange, brown, and pink diamonds are listed in table 4. The blue mar- quise shown here is 3.88 ct; the intense yellow oval weigh 29.16 cts; the two intense yellow diamonds mounted in earrings weigh a total of 12.26 ct. Jewelry courtesy of Harry Winston, Inc.; photo 0 Harold d Erica Van Pelt.

nm (theedge of the green).The resulting blue color, quence of physical phenomena are summarized in which can be produced with boron concentrations table 2. as low as one part per million (Nassau, 1975b),may be quite intense (figure 5). The Hope diamond is Interference Effects from Thin Films. Interference the most famous example of a blue diamond. There phenomena occur when two rays of light travel is no known commercial treatment that would along the same path or in closely spaced parallel affect band-gap coloration. paths. If these rays, or light waves, vibrate in phase, the wave crests reinforce each other to produce COLORS THAT ARISE FROM bright light (constructive interference).If the light PHYSICAL PHENOMENA waves vibrate exactly out of phase, they cancel All of the colors discussed thus far in this series each other to produce darkness (destructive inter- have been due to the absorption of light. But as the ference). introduction to part 1 pointed out, other causes of Iridescence, the most common interference color are possible. In some gem materials, physical phenomenon, occurs when light passes through a properties such as inclusions or lamellar texture thin transparent film that has a different index of can influence the hue. In this next section, we will refraction from the surrounding medium (e.g., a explore how light interference, diffraction, and thin film of air in "iris quartz"). Rays reflected from scattering can interact with these physical fea- the bottom of the film will travel beside waves tures to create colors in gem materials. These reflected from the top of the film. At certain processes are rarely related directly to the chemis- wavelengths, dictated by the thickness and the try of the stone, but rather are connected to the index of refraction of the film, the rays vibrate out texture or internal arrangement of the mineral. of phase and the corresponding colors are removed The various colors obtained in gems as a conse- from the reflected light through destructive inter-

86 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 ference. The remaining wavelengths produce the TABLE 2. Physical phenomena and examples of the colors familiar colored effects that appear when a drop of they cause in various gem materials. oil expands as a thin film on water. The possible colors in iridescence are illustrated in figure 6. Color and None of these colors is a pure spectral color. Process gem material In gemoloey, examples can be found as inter- Interference Various (nonspectral) colors: iris quartz, ference color in cracks (again, "iris quartz"), or in on a thin film iridescent coatings and tarnish, "orient" in tarnish films on oxidized cut stones and sulfide pearls, mother-of-pearl (Nassau, 1975) Diffraction All (spectral) colors: play-of-color opal crystals, such as pyrite and bornite (Nassau, (Darragh and Sanders, 1965), 1975b).Iridescent cracks are sometimes created by labradorite/spectrolite (Ribbe, 1972), heating and rapidly cooling a stone ("quench craclz- some rare andradites (Hirai and Nakazawa, ling"), especially quartz. The color observed in 1982) many pearls is also due in part to interference Scattering Rayleigh Blue: feldspar/moonstone (Lehmann, effects (again, see figure 6). Pearls are constructed scattering 1978), some blue quartz (Zolensky et al,, from concentric alternating layers of aragonite and 1988), some opal (Lehmann, 1978) conchiolin, two substances of different refractive Mie scattering Violet: fluorite, scattering by calcium indices. Incoming overhead light is reflected from microcrystals (Braithwaite et al., 1973) Red: ruby glass, scattering by copper the surfaces between those successive layers. The or gold microcrystals (Nassau, 1983) reflected light interferes with the incoming light to Scattering White: milky quartz (Fritsch, 1985) create delicate iridescent colors, called orient. from structures Mother-of-pearl and some abalone pearls exhibit larger than visible similar interference effects, but the colors gener- wavelengths ally are stronger and less subtle (figure 7). In Presence of Blue: dumorlierite inclusions in quartz addition*to "quench crackling," interference ef- colored (J. Koivula, pers. comm., 1988) fects can also be generated by coating thin films of inclusions Green: nickel-bearing clays in various substances on the surface of a gem. chrysoprase and prase opal (A. Manceau, pers. comm., 1987; Koivula and Fryer, 1984), chromian mica (fuchsite) in Diffraction Effects. Diffraction effects are special aventurine quartz (Lehmann, 1978) types of interference phenomena. The most impor- Orange: hydrous iron oxides in carnelian agate and fire opal (J. Koivula, pers. tant of these for gem materials is that caused by a comm., 1988) regular stacking of alternating layers that have Red: hematite platelets in orthoclase different indices of refraction. This diffraction (Andersen, 1915), hematite or copper platelets in sunstone feldspar (Lehmann, effect produces pure spectral colors, in contrast to 1978), cordierite/"bloodshot iolite" (Gubelin iridescence, which gives rise to colors that are a and Koivula, 1987) combination of several spectral colors (again, see figure 6). Opal is one of the very few gems that can The structure of opal was first revealed with exhibit all colors of the spectrum in a single stone. scanning electron microscopy more than 20 years It is interesting to note in play-of-color opal that ago (Darragh and Sanders, 1965).It is an extraordi- although the pattern may be quite irregular, narily regular stacking of parallel layers of small within each color region the color is homogeneous spheres composed of hydrous silica. Color phe- (see, for example, figure 1 of this article and the nomena occur when the diameter of these spheres cover of this issue). The color of any one patch is less than the wavelengths of visible light. The depends on the orientation of the overhead light conditions for diffraction of a given color are met source; when the stone moves, the color changes, when the distance between two successive layers giving "life" to the opal. If the light emerging from is approximately equal to the wavelength of that one of the color patches is analyzed, it appears to be color divided by the index of refraction of the a pure spectral color, that is, essentially of only one spheres. The exact conditions are described in wavelength. These properties are characteristic of Nassau (1983). Consequently, the diffracted wave- the diffraction effect created by the interaction of length is proportional to the size of the particle. For white light with a regularly layered structure example, the intense red is selected by spheres of (figure 8). about 250 nm in diameter (Darragh and Sanders,

Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 87 Figure 6. The colors produced by interference on a thin film are the same as those observed on this quartz wedge (top left) in polarized light. None of them is a pure spectral color. Notice that the "higher orders" on the right produce mostly pink and green. Interference colors are caused in pearls by light passing through and reflected back by alternating concentric layers of aragonite and conchiolin, which are readily visible on the electron photomicrograph of a pearl section (left). The resulting "high order" interference colors (mostly green and pink) are called orient and overtone (the latter, when they are stronger and homogeneous all over the pearl). They are readily apparent in these black pearl cufflinks (top right). Photomicro- graph courtesy of B. Lasnier, Gemology Laboratory, Nantes University, France. Jewelry courtesy of Harry Winston, Inc. Color photos by Robert Weldon.

1965). The other colors are diffracted by smaller Similar effects are encountered in some feld- spheres, down to 140 nin in diameter. spars belonging to the plagioclase series. These As stated earlier, the color of the diffracted feldspars display regions of color, often violet to light varies with the angle between the direction of green, against a generally black background. Finn- illumination and the direction of observation. The ish "Spectrolite," a variety of labradorite, appears observed wavelength is at a maximum (e.g., red) to show every color of the spectrum. This phenom- when those two directions are perpendicular. enon is called "labradorescence," after the classic When the stone is rocked away from this position, occurrence of these stones on the Isle of Paul, the observed wavelength decreases (e.g., goes to Labrador, although varieties of plagioclase feld- orange; Lehmann, 1978). spars other than labradorite may display this For the more commonplace play-of-color opals, effect. The diffracting objects in labradorescence those with mostly blue and green patches, the are alternating layers, known as exsolution remainder of the incoming light (i.e.,yellow to red) lamellae, of two feldspars with different chemical is transmitted so that an orange coloration is seen compositions. One layer is calcium rich and the in transmitted light. Fire opal, however, probably other is calcium poor. The color created by the owes its yellow-to-red body color (figure 9) to both lamellar structures depends on their respective diffraction and body absorption by Fe3+ -rich sub- thicknesses and indices of refraction (figure 10). microscopic to microscopic inclusions between Another gemstone that occasionally shows diffrac- the silica spheres (J. Koivula, pers. comm., 1988). tion colors is andradite from Hermosillo, Mexico

88 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 Scattering. When the internal structure of the stone is irregular and/or the size of the components is outside the very narrow range needed for diffraction (approximately 100-400 nm), visible light cannot be diffracted. It can, however, still be scattered, the process by which light entering a stone in a given direction is deflected in different directions through interaction with the scattering centers. This creates both striking color effects and optical phenomena. The exact phenomenon depends on the size and shape of the scattering centers. When the scattering centers are smaller than the wavelength of visible light [including down to molecular dimensions) and not regularly distributed, the process is called Rayleigh scattering; when the scattering centers are comparable in size to visible wavelengths, the process is called Mie scattering. (The names are derived from the mathematical theories used to describe scattcr- ing.) A third type of scattering occurs when the centers are larger than visible wavelengths.

Rayleigh scattering. When the incoming light ray encounters randomly distributed objects smaller

Figure 8. The homogeneous color in a patch of opal arises because light rays entering the stone are diffracted by an orderly array of silica spheres and the holes in between them. The diffracted color depends on the size of the spheres (of ter Lehmann, 1978).

ire ', . .'ink and green interference colors Incident light /\ '

(Koivula, 1987). Similar material from Japan has lamellar structures about 100 nm thick (Hirai and Nakazawa, 1982),which give rise to some very rare crystals with patches of color. Diffraction effects probably account for the color phenomena observed in some varieties of agate (e.g., iris agate, fire agate). Diffraction cannot be induced by any known commercial treatment. However, an already existing diffraction color can be enhanced by inducing a dark background (sugar and smoke treatment of opals, for example, as well as doublets], or by reducing the scattering of light in the matrix through impregnation with various kinds of poly- mers.

Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 89 than the wavelengths of visible light, the most so called because light scattered from exsolution energetic radiations -violet and blue- are scat- lamellae creates a bluish white "moon-like" hue tered much more strongly than the others. In fact, [in the best specimens; see figure 1). Moonstone violet light is scattered 16 times more efficiently is actually an alkali feldspar, with alternating than red. As a result, the majority of the orange-red parallel planes of potassium- and sodium-rich light passes through the stone and appears as feldspars forming an assemblage called a micro- transmitted light, whereas violet and blue light is perthite. These component layers in moonstone scattered and can be observed at right angles to the range from 50 to 1000 run (1pm) thick (Lehmann, incident beam. 1978). The thinner layers produce the Rayleigh This phenomenon is familiar to all of us as the scattering. The same colors from scattering can scattering of sunlight by molecules and molecular also occur in plagioclase, and are sometimes called aggregates in the upper atmosphere, which causes "adularescenc6.~Some blue quartz receives its the sky to appear blue in the daytime (scattered color from the scattering effect created by dis- light) and orange-red at dawn and twilight (trans- persed ilmenite inclusions that are approximately mitted light). Examples in gemology are few but 60 nm in diameter (Zolensky et al., 1988). well known. Common opal (potch) contains spheres that are too small and too irregularly Alia Scattering. When the scattering elements are stacked to diffract. Instead, it has a bluish white roughly the size of the visible wavelengths, the appearance called "opalescence," which is due to scattering is best described by the "Mie theory." scattering by the silica spheres. Such an opal This theory has applications in gem materials only indeed also transmits orange light. "Moonstonef1is in those very special cases in which the color is created by metallic inclusions. A common example is provided by some vari- FI~UI~9. This extraordinary opal specimen eties of violet fluorite. This color is generated by from Mesdco shows a difiactim-caused play-of- irradiation, which expels a fluorine atom from the color zoae close to a region of fire opal, which is colored.by a combination of diffractionsad. crystal, leaving partially bonded calcium atoms behind. Over time, the calcium atoms coagulate body a bsqption by FE~+ -containing inclusions. Specimen courtesy of the Paris School of and form small hexagonal platelets about the size Minei; photo 0 Nelly Hariand of visible wavelengths (Lehmann, 1978).Part of the light is absorbed by the calcium crystals and part is reflected. The combined effect of this absorption and reflection is a strong absorption from the green to the red, which leaves a violet transmission window. The position of the bands, and therefore the hue, varies slightly with the size of the metallic particles. Such an effect has been known for a long time in man-made glasses (which are often used as gemstone simulants)."Ruby" glasses are colored by microscopic particles of copper (or gold), and the brown glass used for beer bottles (and to imitate topaz] is usually tinted by metallic oxysulphide precipitates (J. F. Cottrant, pers. comrn., 1987).

Scattering from Structural Components Larger than Visible Wavelengths.When the inclusions are larger than the wavelengths of visible light, they scatter light in all directions, including toward the observer's eye. Unlike the case of Rayleigh scattering, all wavelengths are scattered equally and recombine to produce a white light with a translu- cent milky appearance. This is typical of crystals

GEMS & GEMOLOGY Summer 1988 Figure 10. The schematic drawings illu9trats why vdtious difflaation colors occur ia plagioclase feldspars. The two sets of lameLlar feldspars have different thicfenes$m (dy and dJ and indices of reftacficos (n, and nd; therefore, the light beam will go through in- terfaces 1 mid 2 at differant angles (OÃ and 9J, As a consequence, the beam at interface 2 is retarded dative to the one at iatetface 1, although the wavelengths p the same, and gsneiaQy attenuate each other, For mfc givexi combinaticte of thicknesses, refractive index, and indent warnlength, the beam from interface 2 is exactly one wuveleogth behind the beam of interface 1, so they ewi- bine in a cobereat moaochiosaiatie bearn. The cote of this beam of light is blue for rd- atidy thin lamellw, red for lwei ones. The cameo (by Tiffany Co.) is a lars exampls - of carved zed and blue labiadon'te. The he chdwing is courtesy of the Mineralogical Record; the photo is by Chip Clffrk, ie- printed by permission of HqN. A brams, Inc., courtesy of the Sm'tbsaaicso. InsÂ£1tution

containing pervasive fluid inclusions (such as lets. These effects, beyond the scope of this article, milky quartz), colorless microcr ystals, microfrac- do not affect color. However, for the sake of clarity mes, bubbles, and the like. and completeness, the various kinds of phenomena In some cases, a particular orientation of the (in the gemological sense) have been grouped in scattering elements may produce special optical table 3. With the exception of the alexandrite effects. If they are fibrous, the result is a "white effect, which was discussed in part 1 of this series adulirescencet'or silky sheen, as in gypsum ("satin (Fritsch and Rossman, 1987), all gemstone phe- spar" variety), some malachite, or pectolite. nomena can be understood as the interaction of Chatoyancy or asterism arises when the scattering visible light with particles in a particular size elements are sets of parallel fibers, tubes, or plate- range. The size of the particle can be used as a basis

GEMS & GEMOLOGY Summer 1988 9 1 TABLE 3. Descriptions, causes, and examples of phenomena in gem materials and considerations they require in fa~hioning.~

Considerations Phenomenon Description Cause Examples in fashioning

Iridescence Interference colors on, or in, a Interference of visible light rays, due to Iris quartz, Thin film or Orient stone, like those produced by a the presence of a thin film or thin 'ammolite," structure drop of oil on water structure in, or on, a material of pearls oriented to the different refractive index girdle plane Play-of-color On a given spot, for a given Diffraction of visible light by regularly Opal, feldspar Diffraction Labrador- illumination angle, only one layered structures smaller than the ("spectrolite") layers oriented escence pure spectral color is seen; visible wavelengths parallel to the rotating the stone changes the girdle plane color Adularescence Floating bluish sheen in a stone Scattering of visible light by randomly Feldspar No relation distributed structures smaller than (moonstone) visible wavelengths Chatoyancy One or more lines of white light Scattering of light by oriented parallel Chrysoberyl, Curved surface ("cat's-eye") appearing on a curved surface needle-like or plate-like inclusions or corundum, (not well Asterism (chatoyancy = one ray, like a structures larger than visible quartz, focused on flat cat's eye; asterism = several wavelengths (chatoyancy = one set of diopside surface) rays-up to 6- building a star) inclusions or structures; asterism = multiple sets) Aventurescence Colored metallic-like spangles Reflection of light by large eye-visible Aventurine No relation in the stone, especially obvious plate-like inclusions quartz, feldspar when the stone is rotated in (sunstone), reflected light goldstone glass Change-of- The stone changes color when A major absorption band around 550- Chrysoberyl Observed in color the illumination is switched from 600 nm cuts the visible range in two (alexandrite), all directions, ("Alexandrite sunlight or fluorescent light to transmission windows: one at the blue corundum, better colors effect") incandescent light end (dominating in daylight), the other garnet, spinel, in some at the red end (dominating in fluorite incandescent light)

Â¡'Thi table includes all terms used lor phenomena in gem materials It describes each of the phenomena andshows how similar some 01 them are and how others relate to one another almost in a continuum for classification. There is a continuum in the size Madagascar (Malagasy Republic) and some cor- of the particles between color-creating phenomena dierites ("bloodshot iolite"). Fire opal is colored by (such as adularescence) and phenomena that do not submicroscopic inclusions of iron hydrous oxides affect color (such as chatoyancy). This continuum (J. Koivula, pers. comm., 1988). is emphasized in table 3. When the platelets are large enough to be Some of these phenomena can be induced or distinguished with the naked eye (say, when they enhanced by treatment-especially heat treat- reach 1 mm), they can produce "aventurescence." ment. Basically, the heat precipitates a second This term is used to describe the reflective powder- phase in the matrix, which creates oriented inclu- like appearance of crystalline flakes disposed a sions, which in turn induce chatoyancy or asterism l'oventurra (the term refers to a Murano glass- (Nassau, 1984). first made in Venice, Italy- in which copper platelets have been dispersed at random).In aventurine Presence of Colored Inclusions. The last type of quartz, mica crystals colored green by chromium coloration encountered in gemstones is coloration sparkle when the stone is tilted back and forth. caused by the body color of inclusions in a near- "Sunstonefl can be either native-copper-included colorless host crystal. These inclusions can be labradorite (most commonly), or oligoclase con- extremely small, like the niclzelifero~~sclays that taining red hematite spangles (again, see figure 1). color chrysoprase or the small hydrous iron oxide crystals that make carnelian orange (figure 11). SUMMARY AND CONCLUSION Somewhat larger but still microscopic inclusions In most gem materials, color is caused by selective of hematite cause the color of red orthoclase from absorption of light by different processes. In a few

92 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 instances, phenomenal colors are caused by the interaction of light with certain physical characteristics, such as inclusions, texture, or the structure of component materials. Absorption processes in gemstones can be divided into four broad categories. First, absorption caused by dispersed metal ions explains how an isolated Fez+ metal ion makes peridot green. Second, when certain ions come close enough together, oxygen-to-metal or metal-to-metal charge-transfer transitions are possible, like the 02-+Fe3+ charge transfer that causes the yellow of citrine, or the Fez+-0-Fe^ intervalence charge transfer responsible for the blue in cordierite. Third, color centers represent an extremely varied class of often complex structures or defects that absorb light; for example, carbon vacancies associated with nitrogen aggregates cause an orangy yellow color in diamond. These first three categories give colors that are sometimes easily modified, removed, or enhanced by treatment, usually heat and/or irradiation. In our fourth category, band gaps provide colors that cannot be induced or Figure 11. This m'agnificent Turkoman bracelet was modified by commercial treatment because they made in Central Asia in the 18th century. The car- are directly related to the crystal structure of the nelian is colored by inclusions of hydrous iron oxides. The precious metals owe their white or yellow gem and not to minor amounts of defects or small coloration to preferential re-emission of some visible concentriions of impurities. They could, how- wavelengtl1s. Photo 0 Nelly Bariand. ever, be modified by overriding color-generating processes, but there are no known examples of such a treatment used for gem materials. The origin of color in gem materials is an It is important to keep in mind, however, that a increasingly important topic as more color-alter- single color in a given gem can have more than one ing treatments are used. A detailed understanding cause. In emerald, for example, color can be due to of the origin of color in natural-color gems and the Cr3+, V3+, or both. Table 4 lists the origins of color color-inducing processes involved in the various in most currently available gem materials and enhancement techniques is necessary to provide a illustrates the variety of potential origins for each. better means of separating natural- from treated- This listing refers only to known studies. Conse- color gem materials. Likewise, the origin of color quently, a common color for a given gem might be in some synthetic materials may differ signifi- absent, because no one has yet investigated its cantly from that in their natural counterparts, and cause, whereas the cause of a very unusual color therefore can also be used as a way of distinguish- might be known because it attracted the curiosity ing these two groups. of researchers. As can be seen from our final table, conflicting Ongoing research in a number of laboratories hypotheses on some color varieties and the ab- may lead to results that contradict former origin- sence of documentation on others attest to the of-color assignments. Usually this is because continued existence of substantial gaps in our many of our earlier ideas were "educated guesses" understanding of color. In-depth research is still that were never proven, but nevertheless were needed on some of the most critical gemological often repeated. This probably is also a consequence issues, such as treated colored diamonds and color of the fact that the exploration of the origin of color stability. As new treatments are developed and in a gem material can be a long, difficult, and new color varieties are discovered, the need for expensive process, especially when a color center ongoing research in this area will continue for is involved. many years to come.

Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 93 TABLE 4. Causes of color in most gem materia1s.a

Actinollte Yellowish green to Fez+ in octahedral Burns, 1970 Green (demantoid) Cr3* in octahedral Anderson. 1954- green (nephrite) coordination coordination 55; Stockton and Manson, 1983. Green Traces of Cr3 + Anderson, 1954-55 1984 Almandlne Red Fez* in distorted Manning, 1967a cubic coordination Yellow (topazolite) Various charge- Dowly. 1971; Moore to black (melanite) transfer processes and White. 1971 Amber Blue to green Fluorescence under Schlee, 1984 and dispersed ion visible light in absorption involv- Dominican amber; ing Fe and Ti blue is due to light (Rayleigh) scatter- Anthophylllte and Multicolor "Iridescence," likely Appel and Jensen, ing in Baltic amber gedrite (ortho- ("nuurnmite") diffraction 1987 amphiboles) Yellow to orange to Charge-transfer Nassau, 1975a red to brown processes in large Apatite series Pink F vacancy with a Marfunin. 1979b organic molecules trapped electron Amphibole group (see actinolile. anthophyllite and gedrite. glaucophane, hornblende Dark blue 02- -)MnS+ charge Marfunin, 1979b and pargasite, or tremolite) transfer Apophyllite Green V4+ in distorted oc- Rossman, l974a Andalusite Green and brown, Fez* -0- Ti4+ Smith, 1977 tahedral coordina- pleochroism charge transfer tion Dark green Mn3+ in octahedral Smith et al., 1982 Blue V3+ in octahedral Schmelzer. 1982 (viridine) coordination coordination Andradite Multicolors Diffraction Hirai and Brown Fez + Faye, 1972 Nakazawa, 1982; Koivula, 1987 Azurite Blue Cuz+ in elongated Marfunin. 1979a octahedral coor- Yellow-green Fez+ in octahedral Manning, 1967b. dination coordination 1972 Benitoite Blue Fez + . Not fully proven: charge transfer Burns. 1970 Beryl Dark blue (Maxixe CO; (Maxixe-type) Andersson, 1979 This 1759-ct emerald from the collection of the Bunco de la and Maxixe-type) and NO3 (Maxixe) Republics color centers due in Bogota, Colombia, owes its magnificent color to irradiation to a small amount of Cr3+ in octahedral coordination. Light blue (aqua- Fe2+ in the chan- Goldrnan et al., ec) Photo Harold &> Erica Van Pelt. marine) nels 01 the structure 1978 Darker blue (aqua- FeZ+-O-Fe=* inter- Goldman el al., marine) valence charge 1978 transfer Green: yellow + --)Fe3+ charge G. Rossman, un- blue transfer and Fez* pub data in the channels Green (emerald) Cr3+ andlor V3* in Wood and Nassau, and light green octahedral coor- 1968 ("mint beryl") dination Yellow to orange O%-+Fe3* charge Loeffler and Burns, (heliodor) transfer 1976. Goldman el al., 1978 Red Mna+ in octahedral Shigley and Foord, coordination 1984 Pink (morganite) Mnz+ in octahedral Wood and Nassau. coordination 1968

Calcite Pink co2 + Webster. 1983; Rossman. 1988 Chalcedony Purple Microscopic sugilite Shigley el al., 1987 inclusions Purple Color center similar Shigley and ("damsonite") to that found in am- Koivula, 1985 ethyst Blue to greenish Microscopic to sub- J. Koivula, pers. blue (chrysocolla microscopic inclu- comrn., 1988 quartz) sions of chrysocolla Green Microscopic inclu- A. Manceau, pers. (chrysoprase) sions of nickel- comrn., 1985 ilerous clay-like ma- lerial

'This list of the origin of color in gem materials is based on spectra or explicit discussions as they appear in the literature or have been communicated to the authors One color can be due to a combination of processes, while visually similar colors can have a variety of different causes Within each gem group or subgroup, colors are listed in the order of the spectrum. from violet through purple, blue, green, yellow, arid orange to red, and then pink, brown, black, and white when relevant Dyes and colored coatings can be used on many of these materials, but they are mentioned here only if they are the most common cause ol color in a particular material

94 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 Qpfw(variety or

Gem mat#rial A IM6 We. if '$0~)

Orange to red (car- Submicroscopic to J Koivula, pers. nelian, jasper) microscopic inclu- comm., 1988 sions of hydrous Fe oxides Chrysoberyl Yellow Fe3* in octahedral Loeftler and Burns. coordination 1976 Color-change Cr3* in octahedral Farrell and Newn- (alexandrite) coordination ham. 1965 Chryaocolla Blue Cu'+ in octahedral Lehmann. 1978 coordination Green (tawmawite) Cr3- in octahedral Schmetzer. 1982 coordination Conch "pearl" and Pink Organic pigment Dele-Dubois and shell from the carolenoid Merlin. 1981 lamily Copal (same as amber) Coral Blue An organic pigment Fox et al., 1983 (Heliopora cae- of the bilins family. rulea) helioporobilin Red to pink Organic pigments Delb-Dubois ana from the carotenoid Merlin, 1981 family, at least lor Corallurn rubrurn Black Various organic ma- Rolandi. 1981 terials ol unknown nature Cordlerlte Violet to blue (iolite) Fe7* -0-Fex* Faye el al 1968, charge transfer Smith, 1977. This 32.50-ct "pi~dparadschi~"sapphire owes its beautiful

Goldman et al , color to a combination of Fe3 + - and Cr3+-related absorp- 1977 tions. Jewelry courtesy of Harry Winston, Inc.; photo 0 Red ("bloodshot Hematite andlor Gubelin and ~arolda) ~ricaVan Pelt. iolite") lepidocrocite inclu- Koivula, 1987 sions minor contributions of V3+ and Fe3* in Fe?+ -&Ti4 + Corundum t Purple Schmetzer and octahedral coor- charge transfer co- Bank. 1981 dination I existing with Cr3* in octahedral coor- Pink Cr3* in octahedral G. Rossman, un- dination coordination pub data Color-change Cr3* andlor V3* in Fe2 4 -0-TiÃ Smith and Sirens, Schnietzer et at., charge transfer with 1976; Schmetzer, octahedral coor- 1980 dination in a partic- influence of Fez * -Ã 1987 Fe3* charge trans- ular range of con- fer centration Green Fe3+ in octahedral Schmelzer and Covelllte Blue and orange, Band theory Berry and Vaughan, coordination coex- Bank. 1981 pleochroism 1985 isting with Crocoite Yellow to red 02-+Cr6+ charge Loeffler and Burns, FeZ*-ÃˆTi * charge transfer 1976: Abou-Eid, transfer, Ti3* and 1976 Cr3* in octahedral Cuprite Red Band theory G. Calas, pers. coordination comm., 1984 Yellow O^+Fe3+ charge Schmetzer el al., Diamond Purple, pink to red In natural-color dia- Collins, 1982 transfer 1982; Nassau and monds. attributed Valente, 1987 to a structural de- Fe3* and Ti2 * Schmetzer and fect of unknown na- Bank, 1981 ture A variety of unsta- Schittmann, 1981: In treated pink dia- Collins, 1982 ble color centers of Schmetzer et al., monds, attributed unknown structure 1982, 1983; Nassau to the N-V defect (a and Valenle. 1987 carbon vacancy Fe3* pairs Ferguson and Field- trapped at an iso- ing, 1971 lated nitrogen impurity) Orange to orange- Cr3* in octahedral Schmetzer and brown coordination and Bank, 1981 Blue Band transitions Collins, 1982 color centers; caused by the with a contribution Schmetzer et al., presence of dis- of Fe3 1982, 1983 persed boron atoms Orangy pink Cr3* in octahedral Schmetzer and ("padparadscha") coordination and Bank, 1981 GRt center (neutral Collins. 1982 color centers carbon vacancy) in a colorless dia- Cr4* in octahedral Nassau, 1983 mond irradiated in coordination due to nature or in the lab- Cr-' ' and Mg2+ oratory substituting for two Al3* in the crystal Green Generally GR1 cen- Collins, 1982 structure ter (neutral carbon vacancy), plus de Harder. 1969; Gu- Red (ruby) Cr3* in octahedral fects that absorb in belin, 1975 coordination, with Ihe blue (e.g., N3)

Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 95 Color (variety or Gem material trade name, if any) Cause Reference

More rarely, due to Collins, 1982 Greenish yellow Mn7* in octahedral Rossrnan and very strong green coordination (rare) Malison, 1986 fluorescence under Orange Yellow t pink See Dietrich, 1985 visible light ("green Pink to red Related lo man- Manning, 1973, De transmitter" effect) (rubellite) ganese, generally Camargo and Iso- of the H3 andlor H4 believed to be due tam. 1988 defect (a carbon to Mn3* in octa- vacancy trapped at hedral coordination. an aggregate of sometimes created two or four nitrogen by irradiation atoms) Brown Fez* -)TI"- charge Rossman and Yellow Most commonly Collins, 1982, transfer Malison, 1986 due to Ihe N3 de- Lowlher, 1984 fect, an aggregate Enstatlte Greenish brown Fez * Rossman, 1980 of three nitrogen Green Fe7 * with rninor Anderson, 1954-55 atoms (color cen- Cr3 * ter) at a carbon va- Epidote group (see clinozoisite, epidote, piemontile, or zoisite) cancy Epidote Green and brown, Fe3+ in distorted Burns, 1970 More rarely, due to Collins, 1982 pleochroism octahedral coor- band transitions dination caused by the presence of iso- Euclase Blue Fez * -0-Fe3 * Mattson and Ross- lated nitrogen charge transfer man, 1987 atoms Green Cr3* in octahedral Anderson, 1954-55 Orange Most commonly H3 Cottrant and Calas, coordination center (a carbon 1981 Feldspar group (see labradorite. microcline, oligoclase, orthoclase, or plagioclase vacancy trapped at series) an aggregate of two nitrogen Fluorite Violet Mie scattering on Braithwaite el al., atoms), in natural calcium micro- 1973, Lehmann, and treated orange crystalliles 1978 diamonds Blue Y3+ t F vacancy Bill and Calas, More rarely, origi- Collins, 1982 + 2 electrons 1978 nating from a color 'Emerald" green Sm2 * Bill and Calas. center of unknown ("chrome fluorite") 1978: Rossman. nature 1981 Brown Color center of un- Collins. 1982 Yellowish green Color center con- Bill and Calas. known nature, with taming Y and Ce 1978 various other color associated with an centers adding or- F vacancy ange, yellow, pink, Yellow 0; color center = Bill and Calas. or green 0, substituting for 1978 fluorite Dlopslde Green (chrome Cr3- in octahedral Rossman, 1980 Pink YO, color center Bill and Calas, diopside) coordination. (Y3+ t O;-) 1978 V3* in octahedral Schmetzer. 1982 coordination Color change Y3+-associated Bill and Calas, color center and 1978: Schmetzer et Yellowish green Fez-*-in octahedral Burns. 1970 Smz*. with minor at., 1980 coordination influence of a Dloptase Green Cu7+ in octahedral Lehmann, 1978 Ce3 *-associated coordination color center Dravite Green V3+ generally with Schmetzer and Gahnite and Blue Fez* in tetrahedral Dickson and Smith. ("chrome minor amounts of Bank, 1979 "gahnospinel" coordination 1976 tourmaline") Cr3+, both in octa hedral coordination Garnet group (see almandine, andradite, grossular, hydrogrossular group, pyrope, spessartine, or uvarovite: also rliodolite) Yellow to brown Related to titanium Smith, 1977: Ross- '1 due to Fez * -0- man, as cited in Glass Yellowish green Fep* in octahedral Pye el al , 1984 Tid* charge trans- Dietrich. 1985 (natural) (moldavite, tektite) coordination fer, with those low Brown Fe3+ in octahedral Pye et al., 1984 in iron yellow and coordination those rich in iron Glaucophane Blue FeS t4-~~3+ Smith and Strens, brown charge transfer 1976 Red Fe3* pairs Mattson and Ross- Grossular Green (tsavorite) V3.- in octahedral Gubelin and man. 1984 coordination Weibei, 1975 Elbaite Blue Fe7* in octahedral See Dietrich, 1985 Orange Mn3+ in distorted Manning. 1970 and (indicolite) coordination with (hessonite) cubic coordination: llddlcoatite possible influence Fea Manson and Stock- of some iron-related ton, 1986 charge-transfer processes Gypsum All (alabaster) Color usually due to J. Koivula, pers. dyes exclusively, comm., 1988

Green Fez+ and Ti4 + in Malison, as ciled in except for some octahedral coor- Dietrich, 1985 brown staining due dination. The influ- to hydrous iron ence of various oxides charge transfer processes involving Hematite Gray in reflection, Fe3* Bell el al.. 1975 iron is a distinct red in transmission possibility Hornblende Green to brown Fez+ in various Rossman. 1988 Mnz*-0-Ti4 * Rossman and and pargaslte sites charge transfer Malison, 1986 Howlite Blue Dyes exclusively

96 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 Hydrogrossular Green ("Transvaal Cr3* in octahedral Manning and group lade") coordination Owens, 1977 Pink Mnz* in octahedral Manning and coordination Owens, 1977 Jadeite 'Emerald green Cr3- in octahedral Rossrnan, 1981 (chrome ladeite) coordination Yellowish green Fe3* in octahedral Rossrnan, 1981 cwrdination Violet ("lavender Few -O-Fe3 * Rossman, 1974b jadeite") charge transfer: synthetic is colored Nassau and by Mns* Shigley. 1987 Kornerupine Blue Cr3- in octahedral Schrnetzer, 1982 coordination Green V3* in octahedral Schrnetzer 1982 coordination Kyanite Blue Fez t -0-Ti4 * Parkin el al., 1977 charge transfer. Fez + -0-Fe3 ' charge transfer. Fez* and Fe3* in octahedral coordination can all be involved: with contribution Bosshart et al.. 1982 from Cr3- in octahedral coordination Green V3- in octahedral Schrnetzer. 1982 coordination; Fe3* in octahedral G. Rossrnan. un- coordination pub. data Color change Cr3 ' in octa- Bosshart et al., hedral coordination 1982

Labradorlte " Mullicolors Diffraction of light Ribbe, 1972: by the internal Lehrnann. 1978 lamellar structure Red (in the material Subrnicroscopic Holrneister and Irorn Oregon) metallic copper Rossrnan, 1985b particles Green and orange- Could be Cu * Holrneister and These freshwater baroque and round saltwater cultured pink, pleochroisrn IVCT or CuO pairs Rossrnan, 1985b pearls exhibit the delicate iridescent color (here mostly Blue Fez * -0-Fe3 * Arnthauer and pink) that gemologists call orient. Photo 0 Harold ei) Erica charge transfer Rossrnan. 1984 Van Pelt. Lazurite Blue (lapis lazuli) 83 (charge Loefller and Burn- transfer) 1978 Orange to red (lire Microscopic to sub- J Koivula, pers Pink Mn2+ in octahedral Faye, 1968 opal) microscopic inch- comm . 1980 cwrdination, sions of iron hy- Mna* in octahedral Marfunin, 1979a drous oxides coordination Green Microscopic to sub- Koivula and Frypr Liddlcoatite (see elbaite) (prase opal) microscopic nickel- 1984 Malachite Green Cuz* in octahedral Marlunin, 1979a iferous clay-like in- coordination clusions Maw-sit-sit Green Cr31 in octahedral Khornenko and Orthoclase Yellow Fe3- in tetrahedral Hofmeister and (rock) coordination in the Platonov, 1985 coordination Rossman, 1983 kosrnochlor Pink lo red Microscop~chema- Andersen, 1915, Blue (arnazonite) Color center involv- Hofrneister and tite and/or lepido- J Koivula, pers ing Pb3 and Rossman. 1985a crocite inclusions cornm , 1908 slructural water Pearls (oyster) Nephrite (see actinol~le) Body color All colors Charge-transfer Fox el al.. 1903 Oligoclase Blue (moonstone) Rayleigh scattering Lehrnann, 1978 processes in traces of light by lamellar of porphyrins and structure metalloporphyrins Red (sunstone) Red lepidocrocite J. Koivula, pers, Green High proportions of Fox et al., 1983 or hematite cornrn., 1988; metalloporphyrins. platelets give the Lehrnann, 1978 apparently involving aventurescence lead and zinc Ollvine group: Pink Less total porphyrin Fox el al , 1903 Forsterite-fayalite Yellowish green Fe7- in octahedral Loeffler and Burns, than green series (peridot) coordination 1976 Orient and Pink and green Interference colors E. Fntsch. unpub Green (peridot, the Fez+ with minor Anderson, 1954-55 overtone usually data material from amounts of Cr<-'in Pectolite Blue Cu2+ in octahedral Koivula. 1986a Hawaii) octahedral coor- coordination dination ' Phosphophylllte Bluish green Fe2 * G. Rossman. un- Opal Multicolors (play-01- Diffraction by the Darragh and pub. data color opal) regular stacking of Sanders, 1965 Piemontite Purplish red Mn3* in octahedral Burns. 1970 silica spheres coordination

Color in Gems, Part 3 GEMS & GEMOLOGY Plagloclase series Blue Color center involv- Hofmeister and Smoky (smoky Color center related Partlow and Cohen, ing Pb and water Rossman. 1986 quartz) to the Al3+ impurity 1986 Yellow W+ in tetrahedral Hofmeister and Pink (rose quartz) Charge transfer be- Cohen and Makar, coordination and Rossman, 1983 tween a substitu- I985 Fez* in octahedral tional Tid* and an coordination interstitial Ti3+; unstable color Maschrneyer and Pumpellyite Green Fe^ -0-Fe3 + G. Rossman, un- (chlorastrolite) charge transfer pub. data center 0- Lehmann, 1983 plus FW* ion bridging between substitu- Brownish red Fez* in distorted Manning, 1967a PyroPe tional aluminum cubic coordination and substitutional Red Fez+ in distorted Anderson, 1954- phosphorus atom; cubic site plus 55; Manning, 1967a dumortierite inclu- Applin and Hicks, Cr3* in octahedral sions 1987 coordination White (milky quartz) Scattering of light Fritsch, 1985 Color change (in V3+ and/or Cr3+ in Schmetzer et al., by inclusions larger pyrope and pyrope- octahedral coor- 1980 than the visible spessartine) dination wavelengths Pyrope-Almandlne Reddish purple Fez+ in distorted Manning, 1967a Rhodochroslte Pink to red Mn2+ in octahedral Rossman, 1988 (rhodolite) cubic coordination coordination Pyroxene group (see enstatite, diopside, jadeite, kosmochlor in maw-sit-sit,or Rhodollte (see pyrope-almandine) spodumene) Rhodonlte Pink Mnz* in octahedral Marshall and Quartz Violet to purple 02-->Fed+ charge coordination, with Runcinam, 1975 (amethyst) transfer, due to irra- minor Fez ; diation Mn3+ in octahedral Gibbons et al., Blue Inclusions of blue C. Fryer, pers. coordination 1974 dumortierite comm., 1988 Rutile Blue (synthetic Band transition due G. Rossman, un- Inclusions ol Gubelin and rutile) to the presence of pub. data

tourmaline Koivula, 1987 Ti3 + Inclusions of Zolensky el al., Scapolite series Various colors Due to color cen- Marlunin, 1979b ilmenite of a diame- 1988 ters related to irra- ter smaller than vis- diation of Cl. ible wavelengths Cog- or SCI- Green ("greened Fe2 + Nassau. 1980 groups present in amethyst" or the large voids of prasiolite) the crystal structure Green (aventurine Chromian mica Lehmann. 1978 Scheellte Yellow Fe Not proven: quartz) (fuchsite) inclusions Gunawardene, 1986 Greenish yellow Color center Nassau and Prescolt, 1977 Serpentine Green (williamsite) Cr3+ around chro- J. Koivula, pers. mite inclusions comm,, 1988 Yellow to orange 02- +Fe3* charge Balitsky and (citrine) transfer Balitskaya, 1986 Shattuckite Blue Cu2 + Fleischer, 1987 Various AP+-re- Samoilwich el al., Shell Pink (see lated color centers 1969 conch shell) Black A violet organic Fox el al., 1983 pigment, hal- iotiviolin, has been recovered from the shell of the black Pink rhodochrosite and yellow willemite contrast in hue, abalone, Haliotis although they are both colored by Mn2+. Different coor- cracherodii dination of the Mn2+ ion is the clue here: octahedral in rho- Blue Fez + -O-T~- Rossman et al., docrosite, tetrahedral in willemite. Photo by Robert Weldon. charge transfer, 1982 probably similar to blue kyanite Yellow Fe3+ or Cr3* in Rossman et al., tetrahedral coor- 1982 dination Brown Fe features of yel- Rossman el al., low sillimanite plus 1982 inclusions of iron- rich phase Sinhalite Brown 0Z-+Fe3* charge Farrell and transfer and Fez+ Newnharn, 1965 in octahedral coordination Smithsonlte Blue-green Cup+ G. Rossman. unpub. data Pink CoZ * G. Rossman, unpub. data Sodalite Blue Interstitial oxygen Pizani et al., 1985 ion 0 - near Al or Si Pink (hackmanite) Unstable electron Pizani et al., 1985 substituting for Cl- in a tetrahedron of Na* ions

98 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 Spessartine Orange Mn2 in distorted Manning, 1967a cubic coordination Sphalerite Yellow to black Iron + sulfur Madunin, t979a charge transfer Green Co2+ in tetrahedral Marfunin, 1979a coordination

Spinel group (see gahnite and gahnospinel, or spinel)

Spinel Violet to purple Cra+ in octahedral G, Rossrnan, un- coordination and pub, dala Fez+ in tetrahedral coordination Cobalt blue Co2+ and Fez* in Shigley and tetrahedral coor- Stockton. 1984 dination Bluish green Fe3+ and Fe7* in G. Rossman, un- tetrahedral coor- pub. data dination Green (synthetic Cr3* in octahedral Vogel, 1934 spinel) coordination Pink to red Cra- in octahedral Vogel. 1934; coordination Anderson. 1954- 55; Surnin, 1950 Wulfenite is an intrinsically colorless mineral (left),but it Spodumene Purple to pink Mn3* in tetrahedral Hassan and Labib, commonly acquires an orange coloration when impurity (kunzite) coordination. 1978; chromium atoms produce 02-+Cr6+ charge transfer (center Mn3+ in octahedral Cohen and coordination Janezic. 1983 bottom), which is the intrinsic cause of color in cmcoite Emerald green Cr3* in octahedral Cohen and Janezic, (right). In contrast, blue wulfenite (top) is said to get its (hiddenile) coordination with 1983 color from Mo4+. Photo by Robert Weldon. also unstable Mn4 color center. V3- in octahedral Schmetzer, 1982 coordination Tourmaline group (see'drauite, elbaite and liddicoalile, or uvile) t ' Pink (hexagonite) Mn3 * Paler green Unstable Mn4* in Cohen and Janezic, Tremolite Hawthorne. 1981 octahedral coor- 1983 Tugtupite Pink Color center(s) in- Povarennykh el al., dination plus Fez* volving sulphur 1971 ) Fe3* charge Turquoise Blue Cuz* in octahedral Diaz et al., 1971 transfer coordination Fe3 + in octahedral E. Frilsch, unpub. Uvarovlte Green CrJ* in octahedral Manning, 1969, coordination data coordination Calas, 1978 Greenish yellow to Color center of un- Rossman and Oiu, Uvite Green ("chrome Va* generally with Schmetzer and brownish orange known structure 1982 tourmaline") minor amounts of Bank, 1979. Staurolite Brown Fez+ in tetrahedral Burns. 1970 Cr3* both in octa Schmetzer, 1982 coordination hedral coordination Blue Co-"+ in tetrahedral Cech el al., 1981 Variscite (and Green Cra* presumably in Anderson, 1954- coordination Metavariscite) octahedral coor- 55: Sugilite Purple Mn3* and FeJ*. Shigley et al., 1987 dination Koivula. 1986b presumably in octa- Vesuvianite Green Fe3* in octahedral Manning, 1976, hedral coordination (idocrase) coordination with 1977 Red to violet Cr3+ in octahedral Schrnetzer, 1983 possible influence coordination of Fez * +Fe3 * charge transfer Titanite Green High Fe content Mottana and (sphene) Griffin. 1979 Yellow 02-+Fez* charge Manning. 1977 transfer Green (chrome Cr3* in octahedral Schmetzer, 1982

sphene) coordination Brown Fe7' -)Ti4 + charge Manning, 1977 transfer Pink Mn2+ in octahedral Moltana and Gnlfin, coordination, for 1979 Willemite Yellow Mn7* in tetrahedral G Rossman, un- certain MnIFe ra- coordination pub. data tios, as well as pink Wulfenite Blue Mod* Embrey el al., 1977 carbonate inclu- Yellow to red 07 ->Crb+ charge EdSOn, 1980 sions transfer Topaz Blue Color centers of un- Schmelzer. 1986 Zircon Blue u4* Mackey el al,, 1975 known structure Red Nbd* color centers Fielding, 1970 Green Yellow and blue Petrw, 1977 Zolsite V-1- in octahedral color centers Blue Hurlbut, 1969 (tanzanite- coordination Yellow Color center of un- Petrov. 1977 heat treated) with V3+ in ocla G. Rossrnan, un known nature hedral coordination pub. data Orange ("imperial Yellow color center Petrw. 1977 V3* in octahedral Hurlbut, 1965 topaz") and Cr3* in octa- coordination (treat- hedral coordination ment lurns blue. Pink Cr3* in octahedral Petrov, 1977 tanzanite) coordination Green Cr3- in octahedral Schrnetzer, 1982 Reddish brown Yellow and red Petrov, 1977 coordination ("sherry topaz") color centers Pink (thulite) Mn3- in presum- Marfunin, 1979a Tortoise shell Yellow to brown Charge transfer in Nassau. 1975a ably octahedral co- organic products ordination

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100 Color in Gems, Part 3 GEMS â‚ GEMOLOGY Summer 1988 Phaidon, London. andradite (var. colophonite) and uvarovite. Canadian Min- Gubelin E.J., Koivula J.1. (1987) Photoat-/as of Inclusions in eralogist, Vol. 9, pp. 723-729. Gemstones, ABC Edition, Zurich. Manning EG. (1970)Racah parameters and the relationship to Gubelin E.J., Weibel M. (1975)Green vanadiumgrossulargarnet lengths and covalence of Mn2+- and Fe3+-oxygen bonds in from Lualenyi, near Voi, Kenya. Lapidary Journal, Vol. 29, silicates. Canadian Mineralogist, Vol. 10, pp. 677-688. pp. 402-426. Manning EG. (1972) Optical absorption spectra of Fe3+ in Giinawardene M. (1986) Colombage-Ara scheelite. Gems ei) octahedral and tetrahedral sites in natural garnets. Cana- Gemo/ogy, Vol. 22, No. 3, pp. 166-169. dian Mineralogist, Vol. l l, pp. 826-839. Hassan F., Labib M. (1978) Induced color centers in Manning EG. (1973)Effect of second nearest-neighbour interac- a-spodun~enecalled kunzite. 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Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988 101 Partlow D.l?, Cohen A.J. (1986) Optical studies of biaxial Al- Schmetzer K. (1987) Zur Deutung der Farbursache blauer related color centers in smoky quartz. American Miner- Saphire-eine Diskussion. Neiies Jahrbuch fur Miner- cilogist, Vol. 71, pp. 589-598. dogie, Monatshefte, No. 8, pp. 337-343. Petrov I. (1977)Farbuntersuchungen an Topas. Neues Jahrbuch Schmetzer K., Bank H. (1979) East African tourmalines and fiir Mineralogie Abhandlungen, Vol. 130, No. 3, pp. their non~enclature.Journal of Gemmology, Vol. 16, pp. 288-302. 310-311. Pizani PS., Terrile M.C., Farach H.A., Poole C.l? Jr. (1985)Color Schmetzer K., Bank H. (1981)The colour of natural corundum. centers in sodalite. American Mineralogist, Vol. 70, pp. Neues Jahrbuch fur Mineralogie, Monatshefte, Vol. 11, 1186-1 192. No. 2, pp. 59-68. Povarennykh A.S., Platonov A.M., Tarashchan A.M., Be- Schmetzer K., Bank H., Glibelin E. (1980)The alexandriteeffect lichenko V P (1971) The colour and luminescence of in minerals: Chrysoberyl, garnet, corundum, fluorite. tugtupite (beryllosodalite)from Ilimaussaq, South Green- Neiies Jahrbuch fur Mineralogie, Abhandlungen, Vol. 138, land. Meddelelser om Granland, Vol. 181, pp. 1-12. pp. 147-164. Pye L.D., O'Keefe J.A., Frekchette VD., Eds. (1984) Natural Schmetzer K., Bosshart G., Hanni H.A. (1982) Naturfarbene Glasses. North Holland Publishing, Amsterdam. und behandelte gelbe und orange braune Sapphire. Ribbe PH. (1972)Interference colors in oil slicks and feldspars. Zeitschrift der Deiitschen Cemmologischen Ce- Mineralogical Record, Vol. 3, pp. 18-22. sellschaft, Vol. 31, No. 4, pp. 265-279. Rolandi V (1981) Les gemmes du regne animal: Etude gem- Schmetzer K., Bosshart G., Hiinni H.A. (1983) Naturally- rnologique des secretions des Cnidaires. Revue de Gem- coloured and treated yellow and orange brown sapphires. mologie a.fx., Vol. 66, pp. 3-9. Journal of Gemmology, Vol. 18, No. 7, pp. 607-621. Rossman G.R. (1974a) Optical spectroscopy of green vanadium Shigley J.E., Foord E.E. (1984) Gem-quality red beryl from the

apophyllite from Poona, India. American Mineralogist, Wah-Wah Mountains, Utah. Gems fs) Gemology,-. Vol. 20, Vol. 59, pp. 621-622. No. 4, pp. 208-221. Rossman G.R. (1974b)Lavender jade. The optical spectrum of Shigley J.E., Koivula J.I. (1985)Amethystine chalcedony. Gems Fe^ and Fe2+-Fe3+ intervalence charge transfer in jad- e) Gemoloxy, Vol. 21, No. 4, pp. 219-223. eite from Burma. American Mineralogist, Vol. 59, pp. Shigley J.E., ~oivulaJ.l., ~r~erC.W. (1987)The occurrence and 868-870. gen~ologicalproperties of Wessels Mine sugilite. Gems e) Rossman G.R. (1980) Pyroxene spectroscopy. In C. T. Prewitt, Gemology, Vol. 23, No. 2, pp 78-90. Ed., Reviews in Mineralogy, Vol. 7: Pyroxenes, Mineralogi- Shigley J.E., Stockton C.M. (1984) "Cobalt-blue" gem spinels. cal Society of America, Washington, DC, pp. 93-113. Gems e) Gemology, Vol. 20, No. 1, pp. 3441. Rossman G.R. (1981) Color in gems: The new technologies. Smith G. (1977)Low temperature optical studies of metal-me- Gems s) Gemology, Vol. 17, No. 2, pp. 60-71. tal charge transfer transitions in various minerals. Cana- Rossman G.R. (1988)Optical spectroscopy In F. C. Hawthorne, dim Mineralogist, Vol. 10, pp. 500-507. Ed., Reviews in Mineralogy, Vol. 18: Spectroscopic Smith G., Strens R.G.J. (1976)Intervalence transfer absorption Methods in Mineralogy and Geology, Mineralogical Soci- in some silicate, oxide and phosphate minerals. In R. G. J. ety of America, Washington, DC, pp. 207-254. Strens, Ed., The Physics and Chemistry of Minerals and Rossrnan G.R., Mattson S.M. (1986) Yellow, manganese-rich Rocks, John Wiley & Sons, New York, pp. 583-612. elbaite with manganese-titanium intervalence charge Smith G., Hilenius U., Langer K. (1982) Low temperature transfer. American Mineralogist, Vol. 71, pp. 599-602. spectral studies of Mn3+-bearing andalusite and epidote Rossman G.R., Qiu Y. (1982) Radioactive irradiated spodu- type minerals in the range 30000-5000 cm- 1. Physics and mene. Gems e) Cenwiogy, Vol. 18, No. 2, pp. 87-90. Chemistry of Minerals, Vol. 8, pp. 136-142. Rossman G.R., Grew E.S., Dollase W.A. (1982) The colors of Stockton C.M., Manson D.V (1983) Gem andradite garnets. sillimanite. American Mineralogist, Vol. 67, pp. 749-761. Gems a) Gemology, Vol. 19, No. 4, pp. 202-208. Samoilovich M.I., Tsinober L.I., Kreishop VN. (1969) The Stockton C.M., Manson D.V. (1984) Editorial forum: "Fine nature of radiation-produced citrine coloration in quartz. green" demantoids. Gems e) Gemology, Vol. 20, No. 3, Soviet Physics- Crystallography, Vol. 13, pp. 626-628. p. 179. Schiffmann C.A. (1981) Unstable colour in a yellow sapphire Sumin N.G. (1950) Accessory elements in spinels. Trudy from Sri Lanka. Journal of Gemmology, Vol. 17, No. 8, pp. Mineralog. Museya, Akad. Nauk S.S.S.R., No. 2, pp. 615-618. 113-123. Schlee D. (1984)Ungewohnliche Farbvarianten des Baltischen Vogel l? (1934) Optische Untersuchungen am Smaragd und Bernsteins: Blau, grau, orange und "gold" als Folge von einigen anderen durch Chrom gefarbten Mineralien. Rissesystemen. Bernstein-Neiiigkeiten, Stuttgarter Be- Neues Jahrbuch fur Mineralogie, Geologic und Paleon- itrdge zur Naturkunde, Series C, No. 18, pp. 2-8. tologje, Vol. A68, pp. 401438. Schmetzer K. (1982)Absorptionsspektroskopie und Farbe von Webster R., rev. by B. W. Anderson (1983)Gems, Their Sources, V3+-haltigennatiirlichen Oxiden und Silikaten-ein Be- Descriptions and Identification, 4th ed. Butterworths, itrag zur Kristallchemie des Vanadiums. Neues Jahrbuch London. fur Mineralogie, Abhandl~ingen,Vol. 144, pp. 73-106. Wood D.L., Nassau K. (1968)The characterization of beryl and Schmetzer K. (1983) Crystal chemistry of natural Be-Mg-Al- emerald by visible and infrared absorption spectroscopy. oxides: Taaffeite, taprobanite, musgravite. Neues Jahr- American Mineralogist, Vol. 53, pp. 777-800. buch fur Mineralogie, Abhandlungen, Vol. 146, pp. 15-28. Zolensky M.E., Sylvester PI., Paces J.B. (1988) Origin and Schmetzer K. (1986)Farbung und Bestrahlungschaden in elek- significance of blue coloration in quartz from Llano tronenbestrahlten blauen Topasen. Zeitschrift der Deut- rhyolite (llanite), north-central Llano County, Texas. schen Gemmologischen Gesellschaft, Vol. 35, No. 112, pp. American Mineralogist, Vol. 73, pp. 313-323. 27-38.

102 Color in Gems, Part 3 GEMS & GEMOLOGY Summer 1988