Magnetism in Gemstones
An Effective Tool for Gem Identification
Ugrandites are referred to as the calcium Garnets. All 3 gem species have calcium in the A site of their chemical formulas. Like Pyralspites, they exhibit a wide diversity of color, particularly among Grossular Garnets. Small amounts of chromium from Uvarovite & Knorringite, and vanadium from Goldmanite, are responsible for the green colors we see throughout the Ugrandite Garnets. Within Ugrandite gems, iron is a weak chromophore. Iron (Fe3+) can create strong magnetism while remaining cryptic (colorless).
Merelani Grossular Garnets
Demantoid: Green Andradites are called Demantoid Garnets. They were first discovered in the Ural Mountains of Russia in the mid-1800’s, and today they are also commercially mined in Madagascar and Namibia. Dispersion of light in these gems is even greater than in Diamond, and transparent green Demantoids are the most valuable Garnets in the world, particularly those with dark green color (Chrome Demantoids) derived primarily from chromium/vanadium. The best Chrome Demantoids can sell for $10,000 per carat. Demantoid colors range from brownish green to yellowish green to mint green to dark green. In the graph above, the lowest graph point represents a dark green Chrome Demantoid.
Demantoids are the only natural transparent green gemstones of any kind that pick up with a magnetic wand. The high magnetic susceptibility is due to iron (Fe3+).
Demantoids are also the only Garnets that are commonly treated, with low heat being applied to drive out the yellow and brown color. Heat alters the valence state of iron from Fe2+ to Fe3+, and may eliminate intervalence charge transfer processes that produce yellow and brown color in Andradite. The trivalent iron (Fe3+) remaining in green Demantoid after heat treatment is yellowish green. Strong green color is due to a small amount of chromium/vanadium that may be present in addition to the iron.
Brown and Yellow Andradite: Many Andradites are brown to reddish brown, and on rare occasion, red. Others are yellow, with the trade name Topazolite. Color is likely due entirely to inter-valence charge transfer (iron-titanium and possibly iron-iron, Fe2+-Fe3+). Some brown and yellow Andradites exhibit slight color change (flashes of red) when viewed under incandescent light. All these warm colors of Andradite are grouped as Brown Andradite on the graph above, and all fall along the Grossular-Andradite line.
"Topazolite" from Arizona
Melanite: Black Andradite is called Melanite or Titanium Andradite, and it is generally opaque with a high luster. Melanite forms a solid solution series with Schorlomite, which is a non-gem Garnet that has titanium in the B site of its chemical formula. Titanium, iron and manganese contribute to the black color through inter-valence charge transfer (Fe2+-Ti4+, Mn2+-Ti4+, and Fe2+-Fe3+). No Melanite graph points are shown on the graph above. It is probable that points fall along the line connecting Andradite and Schorlomite, which is for the most part too high to be visible on the graph. There is a wide variance in magnetic susceptibility of black Garnet crystals tested in our study, varying from paramagnetic Melanite crystals to ferromagnetic crystals with extremely high iron content.
Melanite Rough Crystals from Morocco
Iridescent Andradite: One of the rarest Garnets is Iridescent Andradite, also called Rainbow Andradite. It has been mined in Sonora (Mexico), New Mexico (USA), and Nara (Japan). This is an opaque brown Garnet that reflects rainbow colors at the surface when light hits at just the right angle. The rainbow color is due to interference colors and to light diffraction within stratified growth layers (Hainschwang and Notari 2006). This variety of Andradite has the highest magnetic susceptibility and is closer to pure Andradite than any other variety we tested, with a composition of 96% Andradite and 4% Grossular.
Iridescent Andradite Rough from Japan
Chrome Demantoid (Faceted & Crystal Specimen)
Grossular Graph Points
Hydrogrossular Garnet: Hydrogrossular Garnet is translucent to opaque micro-crystalline Garnet, often containing black inclusions of Chromite. Hydrogrossular Garnet is never fully transparent. Translucent gems can have a milky transparency similar to Prehnite or Chalcedony, which are also micro-crystalline. This massive form of Grossular Garnet is mostly fashioned into cabochons rather than as faceted gems. Color is usually green, but also occasionally pink or red. Hydrogrossular can also be colorless. Rare bicolor stones that show both red and green phases have been called "Watermelon" Garnet. Purple stones also exist. When tested with a pinpoint wand, black Chromite inclusions, as seen in the pink gem below (right), show a stronger magnetic response than the body of the gem.
Like gem Andradite, gem-grade Grossulars are Grandite Garnets. They are in solid solution primarily with Andradite (Fe3+), and only to a minor extent with Uvarovite (Cr3+). We have not yet encountered any gem-grade Grossular Garnets that are Gruvites (primarily a mix of Grossular and Uvarovite). Grossular Garnets have low magnetic susceptibilities compared to all other Garnets. The species as a whole does not pick up with an N-52 magnetic wand, and only Mali Garnets typically show a direct Drag response. As you can see in the graph below, most Grossular graph points fall relatively near the pure end-member, which is magnetically inert, colorless and free of iron (Fe3+) or chromium (Cr3+).
It's interesting that most Grossular graph points also fall well below the Grossular-Andradite boundary line toward Pyrope. This is primarily due to Grossular mixing with Hydrogrossular Garnet, rather than due to mixing with Pyrope as you might guess. We have seen that Pyrope Garnets can have significant Grossular content, but chemical studies have shown that the reverse is not true. Gem-grade Grossular Garnets do not freely mix with Pyrope, and Pyrope content is usually under 5% (Manson/Stockton 1985).
Translucent Pink Hydrogrossular Cab
Daylight and UV Fluorescence
Semi-Transparent green gems sold as Hydrogrossular Garnet are probably mainly composed of Idocrase (Vesuvianite species) rather than Garnet. The gem below (left) was incorrectly represented by the seller as Hydrogrossular. Due to a grainy internal texture, this Idocrase gem is not completely transparent, but this degree of clarity is not seen even in the finest examples of faceted Hydrogrossular from Siberia. The photos below compare semi-transparent faceted Pakistani Idocrase (left) and translucent faceted Siberian Hydrogrossular (right).
Hygrosgrossular is more common in its opaque to semi-translucent form, which is often sold by the trade name "Transvaal Jade" (from the Transvaal region, South Africa). This form, which can at times be a mixture of Hydrogrossular and Zoisite, is fashioned into carvings and cabochons. The rich green cabochons below are not Jade, but they do make convincing natural imitations of Jadeite and Nephrite. These dark green Hydrogrossular gems have higher refractive indices (1.725-1.73) than lighter green Hydrogrossulars, and their graph points plot closer toward the Uvarovite end member, indicating detectable chromium/vanadium content and/or lower hydrous content content.
"Transvaal Jade" is Hydrogrossular Garnet that Imitates Genuine Jade
Green Grossulars range from dark green Tsavorite, to lighter mint-green Merelani Garnets, to light "grass" green gems that have no trade name, to the nearly colorless Leuco Garnets that show just a hint of green. All are regarded here as gradations of the Green Grossular variety. As the graphs below show, Merelani and light green gems tend to plot a bit closer to the pure Grossular end member than dark green Tsavorite gems, apparently due to decreasing iron content that correlates with decreasing chromium/vanadium content.
Tsavorite Graph Points
Merelani, Light Green Grossular, and Leuco Garnet (white) graph points
2 Merelanis, Light Green Grossular, Leuco Garnet
Hessonite: Hessonite Garnet is also occasionally referred to as Cinnamon Stone. These gems typically have granular inclusions and show wavy patterns in the crystal structure when viewed under magnification. Colors range from dark reddish orange to light orange to yellow (rare) to colorless (leuco). Hessonites are moderately to strongly magnetic when floated, and some dark gems may even show a weak Drag response. Magnetic susceptibility has been measured as high as SI 321. As a variety of Grossular Garnet, Hessonite on average contains more iron from Andradite than does the Green Grossular variety.
Hessonite (yellow) Graph Points
& Rosolite (pink) Graph Points
Rosolite: This variety of pink Grossular is also sometimes referred to as "Raspberry" Garnet. The pink graph point above nestled among the Hessonites is Rosolite. The bright pink color of this Garnet is due to a small amount of manganese (Mn3+), which also serves as the primary activator that causes orangey pink fluorescence under long wave UV light. Iron ions that induce magnetic attraction in Rosolite are cryptic. We can
speculate the manganese in pink Grossluars may have originally been divalent (Mn2+), and as in pink/red Tourmaline, over geologic time the Mn2+ may have altered to trivalent manganese (Mn3+) by natural gamma irradiation during radioactive decay of an adjacent chemical element.
Pink Grossulars are primarily mined in the Sierra de Cruces mountains, Mexico, but some are also found in Quebec, Canada. Crystals are usually too small to facet into gems, and transparent gems are extremely rare. Rough crystals are most often translucent to opaque, and often grow over a dark core. Some sources describe the dark core as Andradite Garnet, but our examination of the black cubic core of one Rosolite crystal (shown below left) indicates that the core is likely Grossular. A pinpoint magnetic wand reveals that the dark core is indeed more magnetic than the outer pink crystal, but the measured magnetic susceptiblilty is low (SI 239) and well within the range of Grossular Garnet.
Translucent Rosolite Gem 0.54ct
Beautiful Hessonite crystal specimens like the one below (left) are found in Canada. Also shown below is faceted "Imperial" Hessonite from Canada and Sri Lanka, and yellow and near-colorless Hessonite from Tanzania.
(Jeffrey Mine, Canada)
"Imperial" Hessonite (Canada) &
Near-colorless Hessonite (Tanzania)
Mali Garnet Graph Points
Mali Garnet Suite
The trade name Mali Garnet is used throughout the industry, but it is unfortunate that this Garnet variety was named after its country of discovery. Our study shows that Garnets with Mali composition (averaging 82% Grossular, 18% Andradite) can also be found in other parts of Africa (Nigeria, Tanzania and Madagascar), as well as in Russia, Afghanistan and Brazil, although these gems are never sold as Mali Garnets when the origin is outside Mali, Africa. They are generally sold as Grossular or Grandite. To add to the confusion, varieties of Garnet other than Mali (such as Color Change Garnet) are found within the country of Mali. The Hoover method allows us to uncover this information, which otherwise has not been addressed in published literature. Below are photos of two Garnets with Mali composition that are reportedly from Madagascar and Afghanistan.
Yellow Green Mali
Another unusual Mali found in this study changes color from orangey brown (daylight) to red (incandescent). It is reportedly from Tanzania, and has a high RI (1.771) and a high magnetic suceptibility (SI 746). The red color could be due to intervalence charge transfer between iron and titanium (Hoover, pers. comm. 2011). To our knowledge, red color, the phenomenon of color change, and Tanzanian origin has not been reported for Mali Garnet in any published literature. We have tested two other Malis with red color, one with high Andradite content (it picks up with a magnet) and one at the other end of the Mali range with very low Andradite content. This indicates that the color of this gem is independent of the concentration of iron (Fe3+).
Near-colorless Mali (4.6ct)
Daylight and UV Fluorescence
Red Color Change Mali from Tanzania
(2.7ct) in Incandescent Light
Andradite Graph Points
Malis have more Andradite content than other Grossular varieties, but since most are overwhelmingly Grossular, Mali Garnet is more appropriately classified as a variety of Grossular than as Grossular-Andradite (which is the term currently used by GIA). We have found only a few Malis with RI's above 1.77. The highest (1.784) is the Mali pictured below right. Its high graph point, shown in yellow on the graph below left, shows an unusually high Andradite content of aprroximately 33%.
High RI Mali
(Yellow Graph Point)
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Yellow "Imperial" Hessonite (India)
Grossular Garnet has more color varieties than any other Garnet species. Color spans the spectrum from colorless (transparent) to black (opaque), but no true blue or purple gems are known. Hessonite and Tsavorite Garnets are the most common Grossular Garnets in the gem trade. Due to traces of chromium and manganese, orangey pink fluorescence under long wave UV light is common in many light-colored Grossulars.
Tsavorite is the trade name for the highly-prized dark green Grossulars found in Tsavo National Park, Kenya, but green Grossulars are also found in limited quantities in other regions of the world such as Quebec, Canada, and Utah in the United States. Quite a few color varieties of Grossular (colorless, pink, green, orange) have been found in the Quebec mines such as the famous Jeffrey Mine and Oxford Nickel Mine, but the rough crystals are generally small, and few are faceted as gems. On the African continent, colorless Leuco Garnets and near-colorless versions of Green Grossular, Hessonite and Mali Garnets are found. These Garnets have low iron content and only a trace of color.
Mali Garnet: Mali Garnet gems can be visually stunning due to high dispersion that rivals Demantoid. Classified by most gemologists as Grossular-Andradites, Malis are yellowish green to brown, with dark brown gems having the highest Andradite (iron) content. Though rare, dark green and light "mint" green have also been observed. We also find brownish orange, brownish red, and brownish yellow. Inter-valence charge transfer likely plays a role in color. Due to their higher iron content, most Mali Garnets do not show chromium fluorescence under long wave UV light. This Garnet variety was discovered in the West African country of Mali in 1994, and the first analysis of this Garnet's composition was published by Mary Johnson, et. al. in the Fall 1995 issue of Gems & Gemology.
Pictured below is a large near-colorless Mali Garnet, also at the low end of the Mali range. This low Mali (or high Leuco Grossular) has a refractive index of 1.749, and the magnetic susceptibility is SI 3.47. The spectrometer graph beneath the photo clearly shows the presence of Fe3+ in the absence of chromium/vanadium. The concentration of chromiumvanadium is probably below the threshold needed to produce color, and all of the faint yellow-green color and measured magnetic suceptibility is likely due to iron.
This near-colorless Mali Garnet also shows very weak pink fluorescence with a blue laser light (405nm). In this case, photoluminescent spectroscopy suggests that the fluorecence may not be due to chromium, but instead may be due to iron.
Typical Greenish Yellow Mali Garnets
Brownish Yellow Mali
Dark Green Chrome Mali 5.2ct
Green Grossulars: The most valuable variety of Green Grossular is Tsavorite Garnet (shown below left), whose deep blue-green color is similar to fine Emerald. Like Emerald, Green Grossulars are colored by chromium and vanadium in varying proportions. Gems with a very low concentration of chromium/vanadium can appear light green, as we see in the Merelani Garnet shown below (center). The light "grass" green Grossular on the right is probably colored primarily by iron (Fe3+).
Tsavorite from Tanzania
Closely Resembles Emerald
Hydrogrossular Garnet and Idocrase are chemically very similar, and the two minerals can intergrow within the same stone. Both minerals can be translucent to opaque, and have similar appearance, refractive index, magnetic susceptibility and specific gravity values, making it impossible to distinguish between them using standard instruments. Idocrase is doubly refractive, but it falsely appears singly refractive (like Hydrogrossular) under the polarsicope and refractometer. Translucent green gems such as the two Hydrogrossulars shown below (left and center) cannot be distinguished from translucent Idocrase (shown below right) without sophisticated lab tests. Idocrase on rare occasion may also appear pink.
Andradite has the highest refractive index of all gem Garnets, and due to its high iron content, it is by far the most magnetic of the 3 Ugrandite species. The refractive indices of Andradite are over the limit of a standard refractometer. So how do we plot composition on a RIMS graph without RI readings? The Andradite graph point positions shown below were approximated based on the measured magnetic susceptibility in inferred proximity to the Grossular-Andradite join. This is based on reports by other researchers that Andradite is in solid solution series with Grossular Garnet rather than with Uvarovite.
"Pulga Jade" from California
from South Africa
Green Hydrogrossular is colored by a combination of iron and chromium/vanadium, while pink and red Hydrogrossular derives its color from manganese, as does Rosolite Grossular Garnet.
Bi-color Hydrogrossular "Watermelon" Garnet
Hydrogrossular Garnets contain a hydrous (structural water) component, which lowers refractive index and density. Grossular is in solid solutions series with the Hydro-Garnet species Hibschite, which is a non-gem Garnet that has hydrous content. This simply means hydrogen and oxygen (OH) partially replace the silica component (SiO4) in the chemical formula of Grossular.
The khaki-green graph points located at the bottom of the graph below show that Hydrogrossulars fall well below the Grossular end-member. They fall toward the hydrous end-member Hibschite, which is below the range of the graph (the refractive index of Hibschite starts at 1.67). However, we classify Hydrogrossular as a variety of Grossular, since most gems contain Grossular as the primary component.
"Strawberry" Pink Hydrogrossular
Translucent Green Hydrogrossular
Photo Courtesy of Osirisgems.com
The transparent dark green Chrome Mali shown below also derives its dark rich color primarily from chromium/vanadium. This gem shows typical Grossular-Andradite composition on the RIMS. Although the chromium/vanadium is apparent with a spectrometer, the low concentration of chromium/vanadium in this gem does not elevate the refractive index or magnetic susceptibility, and therefor the chromium/vanadium content is not detectable using the RIMS method.
Hydrogrossular Graph Points
Andradite Garnets are found in a range of colors: brown (common Andradite), red, yellow (Topazolite), green (Demantoid), color change, iridescent (Rainbow Garnet), black (Melanite), gray, and in rare instances near- colorless. Colors other than green may involve iron to titanium (Fe2+-Ti4+) and iron to iron (Fe2+-Fe3+) inter-valence charge transfer, while green color is due to iron (Fe3+, yellowish green) and chromium/vanadium (pure green and bluish green). The concentration of iron (Fe3+) in Andradite can be up to 29% iron oxide (Fe2O3) by weight, high enough to create pick-up responses in all Andradite gems, and too high to permit any UV fluorescence from chromium. Even in such high concentrations, Fe3+ iron sometimes creates only light yellowish-green color in Andradite (Demantoid) when chromium/vanadium is especially low. The average composition of Andradite gems that we tested is 81% Andradite and 19% Grossular.
Most Uvarovite Garnet is found in druse form (tiny crystals across the surface of a rock). Uvarovite crystals are generally too small to be faceted as gems, but bright green druse specimens on matrix are often fashioned into jewelry. Uvarovite was discovered in the Ural Mountains of Russia, as was green Demantoid. The majority of druse gems offered on the gem market are from Russia, although Uvarovite is found in other locations such as Quebec, Finland, South Africa and California.
The Uvarovite species is also referred to as Chromium Garnet. Chromium (up to 27% chromium oxide by weight) rather than iron is the idiochromatic coloring agent, imparting a deep green color and strong magnetism to all examples of this species. Vanadium (from Goldmanite) may also contribute to the green color. Crystals are mostly opaque, with some transparency at the surface. Chromium ions are rarely magnetically detectable in any gemstone, but because chromium is in such high concentration in this species, we encounter high magnetic susceptibility that can be measred. Crystals appear bright red under a Chelsea filter, an indication of the high chromium content, but concentration quenching by high levels of chromium prevents any fluorescence under UV light.
Druse specimens attached to matrix are generally too heavy to show a direct response to a magnet. Although not transparent, the small size of druse crystals permits them to transmit more light than larger Uvarovite crystals, and druse specimens appear brighter green in color. Druse crystals are too small to be measured for magnetic susceptibility with a Hoover balance.
Large Uvarovite Crystal in Matrix
For our study, a large 1.78ct (4mm) crystal from Finland (above left) and a large 3mm crystal from Afghanistan (above right) were tested for composition using the RIMS method. The crystal from Afghanistan was embedded in host rock that appears to contain Quartz and Chromite, a mineral rich in chromium and iron.
As with all Garnet species, pure end-member Uvarovite has not been found in nature. Our Uvarovite crystals have a lower measured magnetic susceptibility (SI 9.09 and SI 9.98) than the hypothetical pure end-member (SI 12.75 on the Hoover Diagram). Uvarovite is known to mineralogists to be in solid solution with Grossular and to some extent with Pyrope. In most cases, very little Andradite (i.e. Fe3+ iron) is present. Therefore we use a Uvarovite-Grossular-Pyrope ternary to plot our graph point rather than the standard Uvarovite-Grossular-Andradite ternary. We extrapolate the over-the-limit refractive index based on proximity to the line connecting Grossular to Uvarovite at the measured level of magnetic susceptibility.
The resulting RIMS composition indicates approximately 71% Uvarovite and 29% Grossular for the crystal from Finland, and a higher Uvarovite content of 77% and remaining 23% Grossular for the crystal from Afghanistan. Both also probably contain a small percentage of Pyrope (i.e. magnesium). On the graph below, you’ll find the dark green graph points of our samples situated below the pure end-member along the Uvarovite (Uv.) and Grossular (Gr.) join. Uvarovite can be referred to as Gruvite Garnet, a blend of Grossular and Uvarovite. The dotted line is used indicate the solid solution between Uvarovite and Pyrope. These Uvarovite specimens are the only Gruvite Garnets found in our Garnet study.
Graph Showing Uvarovite Composition
Hessonite (Imperial Valley, California)
Large Uvarovite Crystal
(4mm, 1.78ct, Finland)
Chemical analyses of Uvarovite samples presented in Deer, Howie and Zussman's 1982 book Orthosilicates indicate that Uvarovite compositions can fall anywhere along the Grossular-Uvarovite join between 50% and 91% Uvarovite. Some Uvarovite specimens were shown to have high Pyrope content, while others had higher Andradite content.
Those researchers (D,H & Z) also indicate that some mineral specimens referred to as Uvarovite are actually high-chromium Grossulars that have Grossular as the primary component. Like Uvarovites, they could be referred to as Gruvites. Such unusual Gruvite Grossulars would have their RIMS graph points within the Grossular tri-section of the Ugrandite ternary along the Grossular-Uvarovite join, with Uvarovite comprising less than 50% of the total composition. No Gruvite Grossular Garnet gems have been encountered in our study of Garnets.
Mint Green Mali
Light Orange "Imperial "Hessonite (Sri Lanka)
Daylight and UV Fluorescence
Color in Hessonite is related to small amounts of manganese and iron, and gem color can at times be nearly identical to Spessartine. The light orange and yellow hues likely correspond to varying concentrations of Mn2+, and to intervalence charge transfer processes involving manganese. Red and dark orange colors may additionally be created or modified by inter-valence charge transfer involving iron (Hoover 2013, pers. comm). Color-saturated orange gems are sometimes referred to in the trade as Tangerine Garnets, Fanta Garnets and Mandarin Garnets. Gomed is a term used for Hessonites found in India.
The trade name for gems with light orange color is "Imperial" Hessonite. As with lighter Green Grossulars, lighter Hessonites plot closer to the pure end member than darker gems due to decreasing iron content. Light orange "Imperial" Hessonites also fluoresce pinkish orange under long wave UV light. Our studies with photoluminescent spectroscopy show that fluorescence is primarily due to trace amounts of manganese, although chromium is also often involved in fluorescence. The absence of fluorescence from manganese in dark orange and reddish orange Hessonites, which tend have higher magnetic susceptibility, could be due to the quenching effect of greater iron content and/or to concentration quenching by manganese.
Near-Colorless Andradite: The near-colorless Andradite pictured below left is documented by the RRUFF Project and is considered to be a very pale Demantoid. All the apparent color is likely derived from iron (Fe3+) rather than from chromium/vanadium. Although we have not tested this gem, it qualifies as the only near-colorless gemstone that would be picked up by an N-52 magnet.
It might seem unlikely that a gemstone with such high iron content as Andradite could be near-colorless, but this is another example of how iron in the valence state of Fe3+ is by itself a weak chromophore. The iron in the near-colorless gem (left) is visible only as a very pale yellowish green. We have not encountered any Demantoid Garnets with so little color. The palest Demantoid that we have tested is shown below right. This tiny light green gem was mined in Quebec, Canada, and derives its pale color primarily from iron (Fe3+). It has the lowest Andradite content of all Andradites tested, with a compostiion of 69% Andradite and 31% Grossular.
(Photo by William W. Pinch, Courtesy of rruff.info)
Iron in Near-Colorless Mali
Translucent Red Hydrogrossular
4.07ct., South Africa
Rosolite Crystal Specimen with a Dark Core
Near-Colorless Grossular and Leuco Garnet: Colorless and near-colorless Grossular Garnets are rare. The two gems pictured below are examples of a near-colorless greenish yellow Grossular (left) and a near-colorless yellowish green Grossular (right), both from Tanzania. Both gems have much less iron and magnetic suseptibility than do Mali Garnets, with compositions near the pure Grossular end-member. Both of these gems fluoresce orangey pink in long wave UV light due to manganese and chromium in the presence of very low iron content. Higher iron content likely would quench fluorescence, and higher chromium content would result in stronger fluorescence.
Leuco Garnets in Daylight and Long Wave UV Light
The concentration of chromium/vanadium and magnanese in all the above near-colorless Grossulars and Leuco Grossulars is too low to contribute to measurable magnetic susceptibility. The low magnetic susceptibilities measured are due to entirely iron (Fe3+), and susceptibility values range from just SI 0.15 to SI 1.27. Magnetic responses range from Weak to Moderate. We have not found any near-colorless Grossulars or Leuco Grossulars that are magnetically inert (diamagnetic).
In all cases, refractive indices of near-colorless and Leuco Grossulars were under 1.735. The compositions of these Grossulars are consistent with compositions of light orange Hessonites and light green Grossulars (see the white Leuco graph points on the graph below).
Leuco Garnet Graph Points
Bicolor Garnets are very rare, particularly transparent Garnets that show distinct lines of color separation, as seen in the bicolor Leuco Garnet from Tanzania pictured below. This Garnet shows bright green Tsavorite color on the left side due to chromium/vanadium, and colorless Leuco color (no color) on the right due to insufficient chromophores. This gem is weakly magnetic on both sides, but when tested with a pinpoint magnetic wand, a slightly stronger magnetic response can be detected on the green side (due to slightly more iron rather than to more chromium/vanadium). Fluorescence under long wave UV light appears strongest on the colorless side that has less iron.
Bicolor Leuco Garnet, Tanzania
"Mint" Green Merelani Grossular
Leuco simply means colorless. All Leuco Garnets in our study originated in Tanzania. In most cases, Leuco Grossulars show some remnant of color, either a faint green tint likely due entirely to chromium/vanadium, or a faint orange or yellow tint due to manganese (Mn2+). The iron (Fe3+) content is likely cryptic, producing no visible color. Some Leucos appear completely colorless, and can be up to 99% pure Grossular in composition. Completely colorless Garnets are weakly magnetic due to the low level of cryptic iron.
The three Leuco Garnets pictured below contain manganese and chromium/vanadium in such low concentrations that little or no color is apparent. Yet these gems contain enough chromium (from mixing with Uvarovite) and manganese (from Spessartine) to fluoresce orangey pink under UV light. Most of the Leuco gems tested in our study - whether colorless, faint green or faint orange/yellow - show strong fluorescence under long wave UV light. Fluorescence in Leuco Garnets with a faint orange tint is primarily due to manganese, while fluorescence in gems with a faint green tint is primarily due to chromium.
Yellowish Green Near-Colorless Grossular
Greenish Yellow Near-Colorless Grossular
Light Green Grossular
(Crystals under 0.5mm, Russia)
Large individual Uvarovite crystals are quite rare. These larger crystals (over 1mm) are very dark green in color, mostly opaque and very rarely faceted. Such crystals show a Drag response due to high chromium content, and single crystals under 1ct in weight can show a Pick-up response. Some very rare crystals over 10 mm in size have been found in Finland, and relatively large crystals are occasionally seen from Russia and more recently from Afghanistan.
(Crystals up to 1mm, Russia)
All gem Ugrandites (except Uvarovite and possibly Melanite) can be referred to as Grandites, with major mixing in varying proportions of only 2 end member species, Grossular and Andradite.
Both green and pink Hydrogrossulars fluoresce pink under long wave UV light. A trace amount of chromium is the primary fluorescence activator in green Hydrogrossular, and a trace amount of manganese is the primary activator of fluorescence in pink Hydrogrosular. The iron (Fe3+) that causes all of the magnetic attraction in pink and red Hydrogrossulars is probably cryptic, contributing nothing to daylight body color. The pink Hydrogrossular below has the lowest refractive index (1.70) of any Garnet gem tested in this study.
Since iron content is relatively low in green Grossular Garnets, orangey-pink fluorescence under long wave UV light due to chromium (and sometimes manganese) can be seen, and is especially visible in gems with lighter green color. Although vanadium rather than chromium or iron is likely the primary coloring agent in most Green Grossulars, only the chromium portion causes LWUV fluorescence. Stronger fluorescence is seen in lighter green Grossulars because the concentration of iron and/or vanadium is lower.
Dark Green Tsavorite Garnet
Daylight and UV Fluorescence
Light Green Andradite
Our study has turned up a number of Mali Garnets from Mali that have unusual color, and we find that color variation (near-colorless, yellow, light green to dark green, brown, orange, red) is broader than what is generally reported for Mali Garnet. The refractive index range (RI 1.747-1.784) we found is also higher than what was noted by Mary Johnson (RI 1.752-1.779). On average, Malis have a refracrive index of apporoximately RI 1.76 and a magnetic susceptibility near SI 600, although magnetic susceptibility in Malis at the lowest end of the RI range can be half as much as the average.
Chrome Malis are very rare. These Malis are more pure green than yellow green, and derive most of their color
from chromium/vanadium rather than from iron (Fe3+). The spectrometer graph below left shows both iron and chromium/vanadium in the medium green Chrome Mali from Brazil shown below right.
Chrome Mali, Brazil
Iron and Chromium/Vanadium in Chrome Mali
The "lime" green Mali pictured below is colored by both iron and chromium/vanadium, and shows strong pink fluorescence when a blue laser (405nm) is applied. The fluorescence is due to chromium. As a rule, Mali Garnets contain too much iron to permit fluorescence under UV light. We have not found fluorescence in any Mali Garnets which have a refractive index higher than 1.75. This unusual Mali has a refractive index of only 1.749, lower than what is generally reported for Mali Grossular, but higher than for all Green Grossular Garnets we tested. The magnetic susceptibility is also at the lowest end of the range at SI 291. We can classify this gem either as a low Mali Grossular or as a high Green Grossular, as the solid solution series among Grossular Garnets is continuous and unbroken. We find it surprising that the relatively high iron content of this gem does not quench all fluorescence from chromium.
"Lime" Green Mali
Daylight and UV Fluorescence