Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Color Change Garnets
"Champagne" "Peach" Brown Gold
Green Purple Blue Pink
Although it has not been described in other gemological literature, some Color Change Garnets can show up to four different colors under visible light (as opposed to UV light) depending on the type of visible light applied. An example is the Garnet pear pictured below. This gem changes from purple (daylight) to gray (fluorescent light) to green (white LED light) to pink (incandescent light).
Blue Color Change Garnet
in Daylight (Madagascar)
Blue Color Change Garnet
in Incandescent Light (Madagascar)
Results of out Garnet study suggest that most Color Change Garnets with orange daylight color fall in the Spessartine trisection, and we classify them as Color Change Spessartines. An extreme example of a Color Change Garnet that is primarily composed of Spessartine is the brownish orange trillion pictured below. This gem is represented on the graph above by the far right graph point labeled #3. The major composition of this Color Change Garnet overlaps that of standard orange Spessartine gems, but unlike typical Spessartine, the color changes to red under incandescent light.
We classify this extreme example as a Color Change Garnet, but it could also be classified as a rare example of Spessartine Garnet that is chromium-bearing. The presence of chromium enables the color change.
Chromium can be detected in this gem as red fluorescence under blue laser light (405nm), and chromium is also clearly detected with a spectrometer. Fluorescence in Garnets is usually restricted to the two allochromatic species Pyrope and Grossular, and as far as we know, fluorescence in idiochromatic Spessartine has never been reported. A similar example of chromium-bearing Spessartine from Madagascar was reported in 2002 (Schmetzer, et. al. 2002), but fluorescence in that gem was not described.
Green Color Change Garnet
Orange Color Change Garnet
Color Change Pyropes
The seven Color Change Pyropes shown in the above photo are ordered from left to right to show increasing magnetic susceptibility due to increasing Spessartine content. Because most colors are controlled by low concentrations of chromium/vanadium rather than progressively higher concentrations of Spessartine (manganese), there is no predictable graduation of color. The measured refractive index range for Color Change Pyropes is R.I 1.742-1.768, and the magnetic susceptibility range is SI 14.45- 24.53. Broader R.I. and SI ranges are seen for Color Change Spessartine gems, with RI 1.751-1.792 and SI 24.35- 41.79.
© Kirk Feral 2011, All Rights Reserved. These materials may be duplicated for educational purposes only. No part of this website may be duplicated or distributed for profit, for commercial purposes, or for posting to another website without the expressed written consent of the copyright holder.
Graph of Color Change Garnets (Pyrope & Spessartine)
Blue Color Change Garnet: A striking change from daylight to incandescent light is seen in the .86 ct blue Color Change Garnet from Madagascar pictured below, which changes from blue to pink. This contrast between colors is just as impressive as the color change seen in the finest Alexandrite Chrysoberyl. Below right, we see that blue color is enhanced when viewed under cool LED light.
This Purple Color Change Garnet Shows 4 Colors Under 4 Different Light Sources
Not all Garnet varieties along the Pyrope-Spessartine join are Color Change Garnets. Other hybrids of Pyrope and Spessartine that may not show color change include Pastel Pyrope at the low end, Malaya in the middle, and orange Spessartine at the high end. But Color Change Garnets show the most diverse range of daylight color and incandescent color. Varying amounts of manganese (Spessartine) mixing with small amounts of iron, chromium, and vanadium, result in a rainbow of colors including blue, green, purple, brown, yellow and orange. With the exceptions of blue green and some purple gems, color tone tends to be light or pale. Green Color Change Garnets are the only green transparent gems besides Demantoid Garnets that pick up with a magnet.
The phenomenon of color change is actually found in every gem Garnet species except Uvarovite. In addition to the Garnets that are classified as color change by gemologists, color change can also occur among Almandine, Spesssartine, Andradite, Grossular, Malaya and Pastel Pyrope gems. However, the variety name “Color Change Garnet” is generally reserved for Pyrope-Spessartine Garnets, which at times show the most definitive color change. With compositions that fall along the join between Pyrope and Spessartine in the Pyralspite ternary, we see a nearly continuous isomorphic replacement between Pyrope and Spessartine. This researcher has tested Color Change Garnets with compositions ranging from a low of 30% Spessartine to a high of 89% Spessartine.
Color Change Garnet
Color Change Garnet is an unusual variety of Pyralspite primarily found in African countries such as Madagascar, Kenya and Tanzania, but also in Sri Lanka and Afghanistan. This Garnet variety is found in every color except red and black. The color of Color Change Garnets is modified toward the red end of the spectrum when incandescent light is applied. This is because incandescent bulbs themselves cast a “warm” reddish hue that brings out the red color from minor amounts of chromium within the Color Change Garnets.
Purple Color Change Garnet
Brown Color Change Garnet
The spectrometer graph below illustrates how chromium/vanadium in the presence of manganese creates the blue daylight color and red incandescent color in blue Color Change Pyrope from Madagascar. Absorbance of light in daylight is seen below as the peaks for manganese (Mn2+) and chromium/vanadium (Cr3+/V3+). Transmission of blue light occurs in the valley near 475nm, and transmission of red light occurs in the valley near 700nm. The area of transmission of blue light in the visible portion of the light spectrum is a bit greater than the area of transmission of red light, translating to predominantly blue color in daylight. But when reddish incandescent light is applied, the area of red light transmission shifts to the left and becomes dominant.
Chromium/Vanadium in Relation to Manganese
Creates Blue and Red Colors
In our study, blue Color Change Garnets from Tanzania such as the 0.74ct gem pictured below show less of a complete change in color than do blue gems from Madagascar. The Tanzanian gems change from from blue in daylight to purple (rather than to pink) in incandescent light.
Blue Color Change Garnet from Tanzania
Daylight and Incandescent Light
Blue Color Change Garnet
in LED Light (Madagascar)
Although green Garnet is typically associated with the Ugrandites rather than Pyralspites, green Garnet is occasionally seen as a variation of Color Change Garnet (shown below). The green Garnet trillion below fluoresces bright red under blue laser light (405 nm) due to significant chromium content.
Daylight & 405nm Fluorescence
Green Color Change Garnet
Daylight & Fluorescence
Color Change Spessartine, 1.02ct
Most Color Change Garnet gems with blue or purple color fall in the Pyrope trisection, and these can be classified as Color Change Pyropes. The lowest point on the graph above (#1) is a purple Color Change Garnet pear (pictured below left) that is 69% Pyrope. The blue oval Color Change Garnet from Madagascar pictured below (right) is 63% Pyrope, as represented by point #2 on the graph above.
Blue Color Change Pyrope, .87ct
Purple Color Change Pyrope, 1.04ct
Two Different Species: The graph below shows compositions for 51 Color Change Garnets of various colors. Some Color Change Garnets, such as the blue Garnet from Madagascar shown above, have plot points that fall to the left of the half-way mark along the Pyrope-Spessartine join, and therefore have a composition that is primarily Pyrope. These gems can be classified as Color Change Pyrope, a variety of Pyrope.
More often among Color Change Garnets, points fall to the right of the half-way point, as does the blue Garnet from Tanzania shown above, indicating that Spessartine is the primary component. These gems can be classified as Color Change Spessartine, a variety of Spessartine. Some Color Change Spessartines are nearly pure orange in daylight. GIA currently makes no distinction between Color Change Garnets that are primarily Pyrope and those that are primarily Spessatine, and instead classifies most Color Change Garnets as members of an intermediate species they call Pyrope-Spessartine.
Gems with cooler body colors in daylight (purple, blue, green) show the most dramatic color changes, altering toward warmer colors such as red, pink or purple when incandescent light is applied. Gems with body colors that are already on the "warm" end of the spectrum (brown, yellow, orange, pinkish orange) show changes that are more subtle, becoming reddish orange or reddish pink in incandescent light. Such changes can at times be difficult to see. Examples presented in the photos below depict only daylight colors. The orange Color Change Garnet pictured below looks similar to Spessartine Garnet.
Fluorescence: Most published gemological resources incorrectly describe all Garnets other than Grossulars as non-fluorescing. In fact, many garnets, including most Color Change Garnets, show pink to red fluorescence under blue laser light (405nm). Some Malaya Garnets and Pastel Pyropes can also show fluorescence.
Fluorescence in these Pyralspite garnets is due to chromium, and is permitted by a relatively low concentration of iron. In many cases, the presence of chromium can be confirmed by a red Chelsea filter reaction. In other cases, no Chelsea filter reaction can be seen (as in some brown and orange Color Change Spessartine Garnets). In these cases, only trace amounts of chromium are present, and such low chromium concentrations contribute nothing to daylight gem color and nothing to magnetic susceptibility. I suspect longwave UV fluorescence in many Color Change Garnets is quenched by vanadium rather than by iron.