Magnetism in Gemstones
An Effective Tool for Gem Identification
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.
It's interesting that most Grossular graph points above 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 (1.725-1.728) 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 yellowish green gems sometimes called "Kiwi " Garnets, 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. The lighter color apparently correlates with decreasing green color that is derived from a combination of iron and chromium/vanadium. The amount of chromium/vanadium likely contributes nothing to the measurable magnetic susceptibility.
Tsavorite Graph Points
Merelani & Light Green Grossular (light green graph points),
and Leuco Garnet (white graph points)
2 Merelanis, Light Yellowish 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 show moderate to strong magnetic responses when floated, and most dark orange gems under 3ct in weight show a 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 Point)
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+). 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 fluorescence under blue laser light is probably due to chromium. Iron ions that induce magnetic attraction in Rosolite are cryptic.
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 titanium-rich core. Some sources describe the dark core as Andradite Garnet, but a 2017 analysis by GIA shows the core contains approximately 10% Andradite, 23% Morimotoite, 4% Schorlomite, 3% Pyrope, and the remaining 60% mostly Grossular.
Our examination of the black cubic core of one Rosolite crystal (shown below left) is consistent with the finding that the core is primarily Grossular. A pinpoint magnetic wand reveals that the dark core is indeed more magnetic than the outer pink crystal, but the measured magnetic susceptibililty 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 susceptibility (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)
Red Color Change Mali from Tanzania
(2.7ct) in Incandescent Light
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 approximately 33%.
High RI Mali
(Yellow Graph Point)
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Unusual Yellow "Imperial" Hessonite (India)
Grossular Garnet is the Ugrandite mineral species that is most often fashioned into gemstones. It is a diverse species, with more color varieties than any other species of Garnet. Gem colors span 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.
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 green Mali Garnets do not show chromium fluorescence under long wave UV light, but a Chelsea filter reveals that traces of chromium are common in all colors of Mali Garnets, including brown Malis. 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 chromium/vanadium is probably below the threshold needed to produce color, and all of the faint yellow-green color and measured magnetic susceptibility is likely due to iron. The trivalent iron (Fe3+) found in all Ugrandite Garnets is a weak chromophore, and the low levels in Grossular Garnets are insufficient to cause strong green color.
Typical Greenish Yellow Mali Garnets
Brownish Yellow Mali
Dark Green Chrome Mali 5.2ct
Varieties of Grossular
Green Grossulars: The deep blue-green color of Tsavorite Garnet (shown below left) is similar to fine Emerald. Like Emerald, Green Grossulars are colored primarily by chromium and vanadium in varying proportions, although vanadium is generally conidered to be the dominant chromphore. Iron (Fe3+) may also contribute to the green color. Gems with a very low concentration of chromium/vanadium can appear light green, as we see in the Merelani Garnet shown below (right).
Tsavorite from Tanzania
Closely Resembles Emerald
Hydrogrossular Garnet and Idocrase are chemically very similar, and the two minerals can inter-grow 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.
"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 (Mn3+), as does Rosolite Grossular Garnet. Colorless Hydrogrossular does not contain sufficient chromophores (chromium or manganese) to produce color, yet the example below (right) is still moderately magnetic (SI 143) due to cryptic iron (Fe3+).
Bi-color Hydrogrossular "Watermelon" Garnet
Hydrogrossular Garnets contain a hydrous (structural water) component, which lowers refractive index and density. Water content can be 5% or higher. 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
Hessonite (Imperial Valley, California)
"Mint" Green Mali
Light Orange "Imperial "Hessonite (Sri Lanka)
Daylight and UV Fluorescence
Red and dark orange colors may primarily be the result of 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 (Mn2+), 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.
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 susceptibility 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 UV Light
The concentration of chromium/vanadium and manganese 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 iron and 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 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 Yellowish Green Grossular
Both green and pink Hydrogrossulars fluoresce pink or pinkish orange under blue laser light (405 nm), although fluorescence is much stronger in pink hydrogrossular. A trace amount of chromium seems to be the main fluorescence activator in both colors of Hydrogrossular. Divalent manganese (Mn2+) can also contribute to the color of fluorescence. Although Mn3+ creates pink body color, it is not known to be an activator or co-activator of fluorescence. 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 (pink/red) mixing with a bit of manganese (Mn2+, yellow) can be seen (pink + yellow = orange), and is especially visible in gems with lighter green daylight color. Although vanadium rather than chromium or iron is likely the primary coloring agent in most Green Grossulars, only the chromium portion causes the pink/red LWUV fluorescence. Stronger long wave fluorescence is seen in lighter green Grossulars because the concentration of fluorescence-quenching vanadium is lower. Vanadium can quench some or all of the chromium fluorescence in some dark green Tsavorite Garnets.
Dark Green Tsavorite Garnet
Daylight and UV Fluorescence
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 refractive index of approximately 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.
"Lime" Green Mali
Daylight and UV Fluorescence
Translucent White Hydrogrossular
The most valued Grossular Garnet is Tsavorite (shown below left), a dark green Grossular variety found in Tsavo National Park, Kenya. Tsavorite color is primarily derived from chromium and vanadium. The light yellowish green Grossular from Kenya shown below is probably colored primarily by iron (Fe3+) rather than by chromium/vanadium. Such yellowish green Grossulars have been referred to by the trade name "Kiwi" Garnet.
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.
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 N52 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+).
Color in Hessonite is due to 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.
Hessonites with lighter color tend to plot closer to the pure Grossular end member than darker Hessonites, suggesting that darker orange Hessonite colors are partly the result of higher iron content.
Dark Green Tsavorite
Other green Grossulars from light green to dark green 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 other 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.