Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
To learn why there is such tremendous color variation in Garnets, it’s helpful to have a basic understanding of the chemical formula shown below. The letters A and B simply represent sites where ions of specific chemical elements can bond with silicon and oxygen to give a Garnet a chemical composition unique to its particular species.
The A and B sites can be partially filled by calcium, magnesium or aluminum ions, none of which cause color. But coloring agents can also occupy these sites. The four principal metals that can cause color in Garnets by occupying sites A or B are iron (purplish red, brown, yellow), manganese (orange, pink), chromium (red, green), and vanadium (green). Combinations of these chromophores in varying concentrations result in a bewildering variety of color and subtle variations in hue. Charge transfer processes may also play a role in creating and modifying color. When chromium is present, Garnets with low iron content (Grossular Garnets and Pyrope-Spessartine Garnets) also fluoresce pink or red under long wave UV light.
Pastel Pyrope Garnets
Ferrous iron (Fe2+) ions are located in the A site, shown in bold italics on the graph. The chemical formula represents the pure form of Almandine, which is its end-member form. That means, if only iron and aluminum are found in the A and B sites, the gem is pure Almandine. A pure Almandine gem would have a graph point located at the end of the line that runs from Pyope to Almandine, precisely at the apex of the Almandine trisection of the graph (upper corner). However, all Garnets are hybrids of various Garnet species. Gem Almandines generally contain significant amounts of Pyrope, and Almandine graph points are never close to the pure apex point. Minor amounts of Spessartine and Andradite Garnet are also present.
The fact that color intensity of Almandine remains relatively constant regardless of the concentration of iron suggests that color could also be influenced by a charge transfer process such as iron to iron (Fe2+-Fe3+). In addition, intervalence charge transfer involving titanium ions (Fe2+-Ti4+) might also influence color toward darker tones (Hoover, pers. comm. 2012).
Grape Garnet (Almandine from Orissa, India) #10
Almandine Graph Points
Pyrope Graph Points
Spessartine is one of the most sought-after Garnets due to its brilliance and bright color. Orange color is the rarest and most valuable, but red Spessartines are also desirable, and well-cut specimens show more brilliance than red Almandines and red Pyropes. Spessartine colors range from light "fanta" orange to dark "mandarin" orange to orange-red to red. The color of orange gems could possibly be due to a blending of yellow from manganese (Mn2+ and perhaps charge transfer processes involving Mn2+) in conjunction with red from iron (Fe2+ in Almandine). Spessartine derives most of its magnetic susceptibility from manganese (up to 40% manganese oxide by weight), and Garnets with Spessartine as the primary component are the only primary orange gemstones of any kind that pick up with a magnetic wand. Spessartine has the chemical formula shown below.
Violet Anthill Chrome Pyrope & Orange-red Standard Pyrope
The only thing that makes pure Spessartine chemically different from pure Almandine is that manganese occupies the A site instead of iron. Red Spessartine can at times be difficult to distinguish from red Almandine with the naked eye, but only red Spessartine is over the limit (OTL) of measurable refractive index. Magnetic susceptibility measurements (SI) also easily separate the two. Spessartine is the most magnetic transparent gemstone of any type, always surpassing Almandine. As you can see in the graph below, some Spessartines plot near the pure end member at 47.5 magnetic susceptibility. The purest Spessartines are dark "mandarin" orange. Graph point #10 in the graph below is an example of near-pure Spessartine.
Red and orangey-red Spessartines tend to mix with Almandine along the Sp-Al line, while orange and reddish orange gems tend to be in a solid solution series with Pyrope along the Py-Sp line. As you can see by graph point #14 below, Spessartine can contain up to 50% Almandine. The #14 point falls at the midpoint along the line between the Almandine end member and Spessartine end member (midpoint is indicated as .5). The #14 point has 50% of its A site occupied by iron and 50% by manganese.
Spessartine Colors: Light Orange, Dark Orange & Red
Spessartine Graph Points
Pure End Member Species
Chemical formulas for the six species of gem Garnet are shown below. Chromophores are highlighted in color. These formulas represent the chemistry of pure Garnet species, also referred to as end-members. Chromophores (color-producing metal ions) are found at the A site in Pyralspite Garnet species, and at the B site in Ugrandite Garnet species.
Near-colorless Grossular Garnets:
Hessonite, Green Grossular, Mali
Color Change Garnets
Champagne Peach Brown Gold
Green Purple Blue Pink
Most published gemological resources incorrectly describe all Garnets other than Green Grossulars as inert to UV light. In fact, several varieties of Pyrope-Spessartine Garnets of various colors (low-iron Pastel Pyrope, Color Change Garnet and Malaya Garnet), as well as orange, pink, green and colorless Grossular Garnet, can fluoresce under long wave UV light due to chromium and/or manganese.
Although it has not been described in other gemological literature, some Color Change Garnets can show up to four different colors under visible light (not UV light) depending on the light source. 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 this study suggest that most Color Change Garnets with orange color fall in the Spessartine trisection. An extreme example is the brownish orange trillion pictured below and represented by the far right graph point #3 on the graph above. This gem has similar composition as orange Spessartine gems, but unlike typical Spessartine, the color of this gem changes to red under incandescent light.
We classify this as a Color Change Garnet, but it can also be referred to as a rare example of chromium-bearing Spessartine Garnet. Chromium can be detected in this gem as red fluorescence under long wave UV light, and chromium is also clearly detected with a spectrometer. Fluorescence in Garnets is usually restricted to the two allochromatic speces Pyrope and Grossular, and as far as we know, UV fluorescence in Spessartine has never been reported. A single 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
As you can see, the Low anthill Garnet point has a magnetic susceptibility of less than 5, which is the lowest magnetic measurement this researcher has found for any Pyralspite Garnet. Because the gem is very small, it picks up with a hand-held magnet, but most Chrome Pyropes of average size show only a Drag response.
On the other end of the Pyrope range is the High aqua-blue point, a Standard Pyrope that is 52% Pyrope, with most of the remaining composition as Almandine (48%). This gem has an unusually high magnetic susceptibility measurement of 19.71.
On the graph below, the Low gray graph point to the far left represents a 0.25ct Chrome Pyrope anthill Garnet from a Navajo reservation in Arizona. This Pyrope has a composition closer to the pure end member than any other Pyrope tested. It is approximately 88% Pyrope, which means 88% of site A is filled with magnesium. The remainder of site A is filled with calcium (Grossular), iron (Almandine) and chromium (Uvarovite).
Spessartine: Fanta Garnet (trade name)
Spessartine: Mandarin Garnet (trade name)
Orange Color Change Garnet
Purplish Red Almandine
Pure Almandine gems are not known to exist in nature. The average Almandine gem contains approximately 66% Almandine and 34% Pyrope Garnet (66% of site A is iron and 34% is magnesium,). But any particular Almandine gem will generaly fall in the range of 50% to 75% Almandine, with 25-50% of the A site iron ions replaced by magnesium ions. Magnesium is magnetically inert (diamagnetic). The greater the replacement of magnesium by iron, the more magnetic Almandine becomes, and the further to the right toward the pure end-member its graph points plot.
The graph below shows actual plot points for 27 Almadnine Garnets that come from around the world, from the North America to Africa to India. Notice that Almandine graph points never reach the pure end-member at the upper right apex. Mid-point #10 on the graph is the Grape Garnet (Orissa Almandine) pictured above.
Almandine Cabochon Ring
Pure Grossular Garnet would also be colorless and diamagnetic because there are no chromophores in the A and B sites, only calcium and aluminum. In this case, Grossular gems do approach the pure end-member in composition, and examples of pure colorless gems do exist. These are called Leuco Garnets, though in most cases these Garnets retain a hint of color and a measurable amount of magnetism.
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 varying 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 1.742-1.768, and the magnetic susceptibility range is 14.45- 24.53. A broader range is seen for Color Change Spessartine with RI 1.751-1.792 and SI 24.35- 41.79.
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Pure Red Almandine
Star Almandine, Idaho
Star Pyrope from Sri Lanka
Exceptionally purple Almandine with the trade name Grape Garnet comes from the state of Orissa, India. A dichroic (two colors) Almandine that changes from red to bright purple in incandescent light has been recently mined in the state of Tocantins, Brazil. Grape Garnet and Tocantins Garnet can be very similar in color to Rhodolite Garnet, which belongs to a different species. Gem dealers and gemologists often refer to purple Almandines as Rhodolites. It is our view that the term Rhodolite should not be used to describe every Garnet with purple color, but should be restricted to the purple variety of Pyrope Garnet for which it was originally intended, and which has its own distinct range of chemical composition.
Tocantins Garnet (Almandine from Brazil)
The 100% pure end-member Garnets shown above don't exist in nature. As you can see, pure Pyrope Garnet would be colorless, as there are no transition metal chromophores in the A or B sites, only magnesium and aluminum. The graph point for pure Pyrope would plot at zero magnetic susceptibility. Pure colorless gem Pyrope is not known, but rare near-colorless gems do exist. The most familiar Pyrope gems are red and significantly magnetic due to mixing with other Garnet species such as Uvarovite/Knorringite (chromium) and Almandine (iron).
Garnets as Hybrids: The Pyralspites
Almandine is the most common gem Garnet. Most gems have a dark purplish red or reddish purple color (due to Fe2+ iron), but some gems may appear pure red (possibly due to additional chromium content). All have a high concentration of ferrous iron (Fe2+) as the distinguishing feature (see chemical formula below left). An uncommon phenomenon in Almandine is asterism due to rutile needles. These Garnets are given the trade name Star Garnet (shown below center). Star Almandine is the state gem of Idaho, and is also found in Sri Lanka, India and African nations. Shown below right is the apex section of the Almandine Garnet composition graph.
The chemistry of different Garnet species mix and blend while they are being geologically formed, and the chemical elements in the A and B sites interchange in different ratios. This is called isomorphous replacement. The crystal structure of Garnet remains unchanged, while chemical elements in the A and B sites of a pure end-member is substituted with chemical elements from other Garnet end-members in infinite variation to create intermediate varieties with different colors.
Anthill Garnets (shown below) are small rare Pyropes that are cleared out of underground ant tunnels by ants who carry them to the top of their mounds. Gem prospectors collect these tiny Garnets, but they are not commercially mined. Not all anthill Garnets are Chrome Pyropes, and not all Chrome Pyropes come from anthills.
Graph of Color Change Garnets (Pyrope & Spessartine)
A striking change from daylight to incandescent light is seen in the .86ct blue Color Change Garnet from Madagascar pictured below, which changes from blue to pink. This contrast between colors is equal to that of fine Alexandrite Chrysoberyl. Below right, we see that blue color is enhahced when viewed under LED light.
This Purple Color Change Garnet Shows 4 Colors Under 4 Different Light Sources
The gems that gemologists commonly refer to as Pyrope actually represent just one of six different varieties of Pyrope: 1) Standard Pyrope, 2) Chrome Pyrope,3) Rhodolite Garnet, 4) Malaya Garnet, 5) Color Change Pyrope, and 6) Pastel Pyrope. In addition, dense rutile inclusions in Pyrope can result in the phenomenom of asterism in Star Pyrope Garnet, as identified in this study. Very rare Cat's Eye Pyrope is also known. Pyrope gems of all varieties tested in this study range from just 12% Almandine in a Chrome Pyrope (low refractive index), to 48% Almandine in a Standard Pyrope (high refractive index).
Not all Garnet varieties along the Pyrope-Spessartine line are Color Change Garnets. Other hybrids of Pyrope and Spessartine 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 color and color phenomena. 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 saturation tends to be light or pale.
Many Color Change Garnets, including some blue and green Color Change Garnets, show pink to red fluorescence under long wave UV light. This fluorescence is due to chromium, and is permitted by the low concentration of iron. In some cases (as in blue and green Garnets) 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 Garnets). In these cases, UV fluorescence may be due to just a trace amount of chromium that cannot be detected with a Chelsea FIlter. Such traces of chromium contibute nothing to daylight color and contribute nothing to magnetic susceptibility.
The phenomenon of color change is actually found in every gem Garnet species except Uvarovite. However, the variety name “Color Change Garnet” is generally reserved for Pyrope-Spessartine Garnets, with compositions that fall along the southern boundary of the Pyralspite ternary. A nearly continuous isomorphic replacement is seen between Pyrope and Spessartine along this boundary. This researcher has tested Color Change Pyrope-Spessartine gems 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.
Gems with cooler body colors in daylight (purple, blue, green) show the most dramatic color changes, becoming red, pink or purple in incandescent light. 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.
To calculate the percentage of end-members for any particular gem, we measure the length of the boundary line that connects the Pyrope end-member to the Almandine end-member. Then we measure the distance between the graph point and the end-member apex furthest from it, and divide this number into the total line distance to arrive at the percentage composition of the primary end member species.
For example, let's pretend that the length of the Pyrope-Almandine boundary in the graph above is 100 mm long. The high point farthest to the right on the graph above is 73% Almandine and 27% Pyrope. Its graph point would be measured at 73 mm to the right of Pyrope, because it is 73% of the total length of the line between pure Pyrope and pure Almandine. The low Almandine, the far left point above, would be located 50 mm along the boundary, the half-way point, indicating it is 50% Pyrope and 50% Almandine.
"Pyr" is the Greek word for fire, alluding to the fire-red color for which this Garnet species is known. The pure red to orange-red appearance is due to minor amounts of chromium in addition to iron. Pyrope gems can sometimes be indistinguishable in apearance from Ruby gems, which are also colored by chromium and iron. Since there are a number of varieties of Pyrope, we will refer to typical red and orange-red Pyrope gems as Standard Pyrope. Most Standard Pyropes have much lower RI's than Almandines. They contain an average of 74% Pyrope and 36% Almandine. Pick-up responses tend to have low strength relative to Almandine.
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 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 will shift to the left and become dominant.
Chromium/Vanadium in Relation to Manganese
Creates Blue and Red Colors
Blue Color Change Garnets from Tanzania such as the 0.74ct gem pictured below show a less complete change in color 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)
Almandine End Member
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). This green Garnet trillion fluoresces bright red under long wave UV light due to significant chromium content.
Daylight UV Fluorescence
Green Color Change Garnet
Daylight UV Fluorescence
Color Change Spessartine, 1.02ct
Most gems with blue or purple color fall in the Pyrope trisection. 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. These gems can be classified as Color Change Pyropes. Neither of these 2 gems fluoresces under UV light.
Blue Color Change Pyrope, .87ct
Purple Color Change Pyrope, 1.04ct
The graph below shows that some Color Change Garnet points fall to the left of the half-way mark along the Pyrope-Spessartine boundary, 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 brilliant gem 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.