Color-Change Phenomena in Gemstones Under construction - citations and more images coming   Origin: Kenya, 2.54 ct, Photo courtesy of Gene Flanigan, Precision Gem. | |||||||||||||||||||||||||||||||
| Blue garnets? Many sources will state that they don't exist, but consider the stunning images above of a color change garnet from Kenya. These are perhaps more rare than the better known alexandrite chrysoberyl and exhibit a dramatic color change from greenish blue to a purplish red when the lighting changes from daylight to incandescent indoor light. More startling still is the rare find of a such a garnet as an inclusion in diamond! | |||||||||||||||||||||||||||||||
| But, wait! There are also blue garnets which don't change color! At right is a just such a blue garnet sent to me from the collection of Prof. Henry Hänni of SSEF: a non-colorchange blue garnet; a pyrope-spessartine with a distinct vanadium content (V2O3 1.60 wt %). Calculated end-member mol%: pyrope 39.30%, spessartite 39.50%, almandine 1.40%, uvarovite 1.10%, goldmanite 4.90%, grossular 13.80% (H. Hänni, personal comm, 2008). |
 0.66 ct, Bekily, Madagascar. Collection of Prof. Henry Hänni Photo courtesy of: H.A.Hänni. | ||||||||||||||||||||||||||||||
| Color-change in any gem material is a complex interaction between many factors, not all of which are completely understood. Below is an exploration into some of what is known today. As with alexandrite, color change in garnet is in part due to its even balance of transmission wavelength peaks (maxima) in the green/blue vs red spectra, the perception of which is tipped by the type of light source under which the stone is viewed. In daylight which is stronger in the blue green wavelengths, the stone will appear blue; in incandescent lighting which is richer in the red wavelengths, the stone will appear purplish red (see transmission curves ). | |||||||||||||||||||||||||||||||
In color change pyrope-spessartine garnets from Madagascar this effect has been attributed to the presence of vanadium and/or chromium (rare), however the relationship is complex and the correlation is not direct: even garnets with a relatively low vanadium content may show a strong change while others relatively rich in vanadium may show a weak change. This has been explained by the interaction of other elemental variables interacting with the vanadium concentration: "...the observed variations of the color change effect in garnets is generally an effect of chemical variations in concentrations of the trace elements, vanadium and chromium and of the major constituents manganese and iron (M. Krzemnicki, 2001)." See images of color-change Malaya garnet from Bekily, Madagascar. |
 and fluorescent light (bottom) Origin: Tanzania, 1.36 ct Photo courtesy of Gene Flanigan, Precision Gem. | ||||||||||||||||||||||||||||||
Alexandrite-like color change occurs in many different gemstones and is not restricted to either singly refractive or doubly refractive minerals. Along with manmade materials such as flux grown synthetic alexandrite, flame fusion synthetic sapphire sold as imitation alexandrite, synthetic diamond, and glass such as "Zandrite," naturally occurring color change gems include sapphire, spinel, fluorite, kyanite, tourmaline, zircon and of course alexandrite chrysoberyl. To understand color change one must first accept that all color perception is a product of the mind's interpretation of stimulus received from the eyes in response to light's interaction with a material. Perhaps the most elegant summation of the alexandrite effect is that of Dr. Kurt Nassau who writes: " (it) occurs as a psychophysical phenomenon derived from the specific response characteristics of the human eye-brain combination to the different illuminations." The phenomenon is not completely explained by the balance of transmission maxima and illumination type; the full compliment of factors must be considered. Y. Liu states "...color perception is determined by four factors: chromaticity adaptation, the spectral sensitivities of the (eye's) cone photoreceptors, the spectral power distribution of the light source, and the spectral transmittance of the material being observed. A color phenomenon cannot be explained by relying on only one or two of these factors." In an earlier paper published in 1999, Y. Liu elaborated on these concepts in much more detail including defining chromatic adaptation: "....our eyes adjust when observing an object under different light sources, so that the object's colour often appears nearly constant as the lighting changes (such as an apple that appears red under daylight, incandescent light and fluorescent light)." Though the most well known color change is that of Alexandrite, some effects are due to interaction of several mechanisms. Most notably this is seen in alexandrite itself where pleochroism dramatically influences the effect. The change is much more dramatic for light travelling parallel to the a axis of the crystal, therefore the optimal orientation of the table is perpendicular to this axis (Liu, 1995). There are three seemingly differing interpretations of what is known as "color shift. " The original coining of the term is to be found in the 1984 winter issue of Gems & Gemology in which Manson and Stockton of the GIA describe a change in color caused by transmitted light versus that of internally reflected light, without a change in illumination, as observed in color change pyrope-spessartite garnets. The paper's accompanying photographs illustrate the phenomenon, roughly as follows:
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 Colored pencil rendition with apologies to Tino Hammid. Hopefully this abominable illustration will encourage the reader to seek out a copy of the G&G, Winter 1984 to see the real thing: a single garnet pebble showing the original description of color shift: a change in color caused by transmitted light versus that of internally reflected light, without a change in illumination. | |||||||||||||||||||||||||||||||
| Later in Y. Liu's 1999 paper, it states "At GIA-GTL a gemstone that exhibits different hues under different light sources is described as showing a color change when this behaviour is not typical of all samples of that gem varieties (S. McClure, pers. comm., 1998). Certain purple gemstones, such as amethyst, always display different hues under different light sources; these gemstones are described at GIA-GTL as showing colour shift." Lastly, turn to the master of articulating color, Richard W. Wise, in his GemWise Color Change/Color Shift essay in which he proposes a definition which encompasses the aesthetics of color in relation to the phenomenon (quote):
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 Photos used with permission; for image source, contact me) Color shift Malaya garnet from Bekily, Madagascar, 1.97 ct. See Schmetzer et al. Gems & Gemology Winter 2001 for a full description of these Malaya garnets. (daylight at left, incandescent light at right) | |||||||||||||||||||||||||||||||
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