Magnetism in Gemstones
An Effective Tool for Gem Identification
© Kirk Feral
"Verdelite" Tourmaline 4.1ct.
"Rubellite" Tourmaline 8.1ct.
When testing Garnets, we can accurately determine the major composition of individual gems by graphing refractive index and magnetic measurements on solid solution ternary diagrams much like those we show for Tourmalines (see page 1 of Garnets for a Garnet diagram). For a variety of reasons, this method of determining composition is not well-suited to Elbaite and most other Tourmaline species. But we might expect that the compositions of most gem Tourmalines would fall somewhere within the Elbaite-Schorl-Dravite ternary or the Elbaite-Schorl-Tsilaisite ternary. At the present time, the best we can do is to show the measured magnetic susceptibility of each individual Elbaite gem on a bar graph.
Black: This is the most common color of Tourmaline found in nature, but black Tourmalines are seldom faceted as gemstones. Most belong to the Schorl species, which is an opaque Tourmaline that is often black in color. Schorl owes its color and lack of transparency to high concentrations of 3 transition metals: 1) iron within its chemical formula, 2) manganese impurities, and 3) titanium impurities. Titanium contributes color through inter-valence charge transfer (Fe2+-Ti4+). Charge transfer processes involving other ions also contribute to the black color. Black Tourmalines show a Drag response. Quantitative measurements of magnetic susceptibility tell us that opaque black Tourmalines are by far the most magnetic of all the color varieties of gem Tourmaline.
(2.42ct. & 2.23ct., Drag, SI 990 &1150)
Tourmaline species are classified according to chemical compositions that are often unrelated to color or magnetism. Different species generally can’t be distinguished from one another by their color or magnetic responses or magnetic susceptibility measurements. Gems of different species can have the same color, and the transition metal impurities and charge transfer processes that appear in one species may also appear in other species. For example, not all black Tourmalines are Schorl. Some Dravite and Uvite gems are also black. Another example is brown and orange Tourmaline.
Brown and Orange: Dravite is typically a brown or orangey brown Tourmaline colored by iron to titanium charge transfer (Fe2+-Ti4+). The iron is derived from mixing with Schorl. Iron ions in brown Tourmalines are often not magnetically detectable because only very low concentrations of iron are needed to create color through the iron-titanium (Fe2+-Ti4+) charge transfer process. The 3 brown tourmalines pictured below show an Inert (Diamagnetic) response to an N-52 magnet. Their color and lack of magnetic attraction are not necessarily specific to the Dravite species alone. Elbaite, Liddicoatite and Uvite gems can also be brown, presumably due to the same iron-titanium charge transfer process. However, we will assume that all brown Tourmalines shown below are most likely Dravite.
These Brown Tourmalines are Diamagnetic
"Chrome" Tourmalines, "Vanadium" Tourmalines and "Verdelite" Tourmalines can sometimes look alike (see comparison below left). Pale green or mint green "Vanadium" Tourmalines with light colors due to lower concentrations of vanadium can look identical to pale green Tourmalines colored by iron (see the oval and rectangle gems below right). Fortunately, we can use magnetic response to easily separate these Tourmalines whether they are light green or dark green.
"Chrome" Tourmalines and "Vanadium" Tourmalines (Dravite species) of any color intensity usually show an Inert (Diamagnetic) response, or in some cases very weak attraction. Only 2 of 11 "Chrome" Tourmalines tested in our study show slight magnetic attraction. In contrast, green Elbaite Tourmalines of any color intensity caused by iron (Fe2+) show significant magnetic attraction, either via the Direct Method or Floatation Method.
"Chrome" Tourmaline (.86ct., Inert, SI <0) & Verdelite (.79ct., Drag, SI 317)
Blue: Most blue Tourmalines (Elbaite species) are colored by ferrous iron (Fe2+) as well as iron to iron charge transfer (Fe2+ - Fe3+). The iron is derived from mixing with Schorl in solid solution series. On rare occasions, Tourmalines are formed in a geologic environment that is high in copper. The copper will contribute blue color in such Tourmalines, which have been given the trade name "Paraiba" Tourmalines (see page 3).
Magnetic response can identify blue "Indicolite" Tourmaline, which is an indigo blue gem primarily of the Elbaite species. It is colored by iron, and a Drag response is diagnostic for "Indicolite". This is the only blue gemstone of any kind, not just Tourmaline, that will show a Drag response.
Often blue and green colors mix within a single Tourmaline gem. Strictly speaking, only pure indigo blue gems such as the oval gem above (right) are "Indicolite", but pure blue gems are seldom seen. Most dark blue gems referred to as "Indicolite" contain some amount of green, like the gem shown above left. The square gem pictured in the photo below left seems to have equal amounts of blue and green color. It shows a Drag response, but can we call it "Indicolite"? The bi-color oval in the photo below right is predominantly dark blue, but it also has some green and yellow color. It shows only a Moderate response, indicating much lower iron content than what we see in "Indicolite". These are just two of many examples of Tourmalines in shades and mixtures of colors that do not neatly fit any single color variety or trade name.
Greenish Blue Tourmaline
(2.59ct., Drag, SI 304)
Blue & Green Tourmaline
(2.85ct., Moderate, SI 91)
Our study of Tourmaline magnetism uncovered other surprising results. As an example, not all dark blue Tourmalines show a Drag response typical of "Indicolite". The gem pictured below (left) shows only a Weak response when floated, and the gem on the right shows only a Strong response. We can speculate that the dark blue color in the gem on the left may be primarily influenced by inter-valence charge transfer between iron ions (Fe2+-Fe3+), while dark blue color in the gem on the right may be controlled more by the concentration of iron (Fe2+) ions.
Dark Blue Tourmalines
(.92ct., Weak, SI 69 & .911ct., Strong, SI 218)
Tourmaline is a diverse mineral group composed of at least 25 species currently known to mineralogists (Henry, D.J. et. al., 2011). Most gem-quality Tourmalines belong to the Elbaite species, which is allochromatic. Color varieties and trade names of Elbaite gems include green "Verdelite", red "Rubellite", blue "Indicolite", pink Tourmaline and yellow "Canary" Tourmaline. Brown gems often belong to the Dravite species, and black color is typical of the Schorl species. The metal ions that cause color in any particular Tourmaline gem can only be determined by advanced lab tests that include microprobe, laser ablation and spectrometric analysis. However, specific colors have been found by researchers to be indicative of specific metal ions and intervalence charge transfer processes.
Pink Tourmaline 3.5ct.
"Indicolite" Tourmaline 5.3ct.
Green: Green is the most common color of transparent gem Tourmaline. Color ranges from "grass" green to bluish green to "Emerald" green, and color intensity can range from very light to very dark. Green "Verdelite" Tourmaline is colored by dispersed ions of iron (Fe2+, derived from mixing with Schorl) in conjunction with iron-titanium (Fe2+ - Ti4+) charge transfer.
Pale Green (4.1ct., Moderate, SI 69) &
"Verdelite" (4.1ct., Drag, SI 278)
Pale Green (1.15ct., Moderate, SI 117) &
"Verdelite" (.79ct., Drag, SI 317)
Grayish green and blackish green Tourmalines are fairly common. Below are 3 examples. The over-dark green hexagon on the left shows a combination of green and gray colors. The color may be influenced by natural radiation. Studies have shown that lab irradiation of some green Tourmalines can produce gray color (Kurt Nassuau, 1975). This may involve alteration of the valence state of iron from Fe2+ (green) to Fe3+ (gray), as well as to Fe2+-Ti4+ charge transfer.
The magnetic response of the hexagonal gem (left) is Weak. The blackish green trillion in the center shows a Strong magnetic response. These 2 Tourmalines have magnetic susceptibilities that correspond to pale green gems. In fact, if the black component could be removed, these Tourmalines might show only pale green color.
Blackish Green Tourmaline
(5.36 ct., Weak, SI 74)
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The bar graph below shows the magnetic susceptibility (degree of magnetic attraction) of different Tourmaline color varieties in relation to one another. Each column of dots represents a color variety, presented from least magnetic to most magnetic. Dot colors are representative of gem colors, and each dot represents an individual gem's measured magnetism in terms of volume magnetic susceptibility noted as SI (International System of Units) X 10(-6). The SI values range from less than zero (< 0 SI, Diamagnetic response) to SI 547 (Drag response) for transparent gems, and from SI 773 (Drag response) to SI 990 (Drag response) for opaque gems. Gems with mixed colors are not represented on this graph.
Relative Magnetic Susceptibilities of Tourmalines by Color
All Tourmalines tested for this study had refractive indices in the range of 1.61-1.645, with the exception of iron-rich Schorl, which had a higher refractive index range of 1.63-1.67. Elbaite chemically mixes with other Tourmaline species such as Schorl, Tsilaiste, Liddicoatite and Rossmanite in what are referred to as solid solution series (see table below). Dravite is in solid solution with Uvite and Schorl. Some of these species are idiochromatic, such as Schorl (iron, Fe2+), Tsilaisite (manganese, Mn2+), Chromium Dravite (chromium, Cr3+) and Vanadium Dravite (Vanadium, V3+). These are idiochromatic species that have transition metals as part of their chemical formulas. Allochromatic Elbaite is a sodium/lithium Tourmaline species that derives most of its colors from the idiochromatic species with which it blends.
This is not the first table created to show magnetic susceptibilities of Tourmalines. In 1944, researcher A. Sigamony published a paper titled The Magnetic Properties of Tourmaline and Epidote in which he included a table of magnetic susceptibilities for 11 Tourmalines based on color. The values were obtained by measuring loss of weight using an ALNICO magnet and a Curie balance, which produces measurements that are significantly less precise than what we obtain with a neodymium magnet and Hoover balance, which we used for the current study. Also, in the classic 1985 monograph on Tourmalines titled The Tourmaline Group, R. Dietrich published a table of magnetic susceptibilities for 38 Tourmalines based on color. That table is a compilation of data from mineralogists who published from the 1940’s through the 1970’s. Those first limited efforts undertaken decades ago by mineralogists who relied on less sensitive instruments to measure gem magnetic susceptibilities are remarkably consistent with the results of our current study.
(5.25ct., Drag, SI 447)
1.18ct., SI 127 .48ct., SI 313 .81ct., SI 382
"Vanadium" Tourmaline (1.58ct., Inert, SI <0) &
Pale Green Tourmaline (1.15ct., Moderate, SI 117)
Pale Green (5.06, Weak, <20 SI) &
"Verdelite" (5.94ct., Drag, SI 360)
When a handheld spectroscope is applied to the gems above, we see no definitive absorption spectra that might help us separate the 2 species of green Tourmaline. A Chelsea filter or UV light can help us identify "Chrome" Tourmalines, as these gems can glow red or pink. However, these tools may not identify all "Chrome" Tourmalines. Those gems colored primarily by vanadium will not glow red under the Chelsea filter, and gems with very low concentrations of chromium may show no Chelsea filter reaction or red/pink UV fluorescence. In such cases, a handheld magnet may be the only available tool we have to separate these Tourmalines.
A “Chrome” Tourmaline with atypical brownish green or "olive" green color was also encountered in our study, as pictured below. This diamagnetic gem shows pink/red fluorescence under long wave UV light, and a slight reddish glow under the Chelsea filter. Chromium/vanadium is confirmed with a spectrometer as the primary cause of color.
Paler blue colors and shades colored by iron are also found among Tourmalnes. Pale gems that are greenish blue and powder blue can be appropriately referred to as blue Tourmalines rather than as "Indicolite". The magnetic susceptibility of these blue Tourmalines corresponds to the intensity of color. The two gem triads pictured below show a progression of increasing color saturation from near-colorless to dark blue. Magnetic susceptibility (SI) of the gems in each triad also progressively increases from left to right.
.48ct., SI 55 .58ct., SI 145 .59ct., SI 327
Progressive changes in magnetism can also be measured within a single bi-color gem, as depicted in the blue Tourmaline bar below (3.23 ct.). From left to right, the color intensity of this gem decreases from opaque dark blue to colorless. Measurements taken at 8 different points along the length of the 24mm bar show a ten-fold decrease in magnetic susceptibility from SI 781 to SI 78, presumably due to progressively lower iron content. Refractive index also decreases from 1.64-1.65 to 1.63-1.64 due to lower density caused by a lower concentration of the metallic chromophore.
Much of the dark portion on the left side of this bi-color gem is opaque, except at the facet edges where it is thin and transparent. It's likely that the dark blue portion with susceptibilities above 500 has a primary content of Schorl rather than "Indicolite" (Elbaite species). This portion shows a Drag response. It also shows color change (dichromatism). When viewed in intensely bright incandescent transmitted light, the opaque dark blue color becomes transparent lavender (reddish light + blue body color = lavender). Opaque black Schorl has also been reported to appear lavender when cut in thin section. Surprisingly, the colorless portion on the right side of this gem retains enough iron (Fe3+) to show weak to moderate magnetic attraction, indicating the iron is cryptic.
SI: 781 694 599 529 269 156 91 78
Blue Becomes Lavender in
Transmitted Incandescent Light
Moderate Drag Drag
Weak Strong Drag
Indigo Blue "Indicolite"
(.41ct., Drag, SI 382)
Tourmaline Species Mix Together in Solid Solution Series.
The Chromophores for 3 Idiochromatic Species are Highlighted in Red.
Blackish Green Tourmaline Rough
(17.62ct., Strong, SI 286)
Contents of this Section
Color Varieties: Black, Orange and Brown, Blue, Green
Color and Magnetism
Because different Tourmaline species have overlapping refractive indices, densities and magnetic susceptibilites, they cannot be definitively separated from one another using standard gemological tools. Advanced lab analysis is needed to separate Tourmalines by species based on their chemical compositions. For this reason, our study of Tourmalines separates gems primarily by color rather than by species. Our assumption is that most faceted gem-quality Tourmalines are Elbaite, most Chrome Tourmalines and most brown/orange gems are Dravite, and most opaque black gems are Schorl.
The table of ternaries (triangles) below illustrates some of the many possible solid solution series among gem Tourmalines. The dotted line represents the Schorl-Tsilaisite solid solution. Vanadium-Dravite and Rossmanite species are not included on the table.
"Chrome" Tourmaline 5.4ct.
The blackish brown Tourmaline round from Sri Lanka pictured below (left) appears opaque and black in reflected light and can easily be mistaken for black Schorl. Transmitted light (back lighting) reveals that this gem is actually transparent and dark brown. The gem shows a Moderate magnetic response, and we can speculate that this may be due to "surplus" iron that is not engaged in iron-titanium charge transfer. Perhaps for the same reason, the translucent brown rough Tourmaline on the right shows a Weak magnetic response. The magnetism could also be partly due to manganese (Mn2+).
Opaque Black in Reflected Light &
Transparent Brown in Transmitted Light
The 2 brown gems shown below are both strongly magnetic. The oval gem on the left also has yellow and green color, while the pear-shaped gem on the right is yellowish brown. The yellow color in these gems is likely due to manganese, whose presence is suggested by spectrometer analysis. Manganese as Mn2+ can impart strong magnetic susceptibility, as we will show in several other color varieties of Tourmaline.
(1.32ct., Moderate, SI 78)
Yellowish Greenish Brown
(6.65ct., Strong, SI 265)
(2.41ct., Strong, SI 221)
Orange is often a component color of brown Tourmalines. On rare occasions, pure orange gems are also found, as depicted by the 2 gems below. Like brown Tourmaline colored by iron and Fe2+-Ti4+ charge transfer, these orange Tourmalines show no attraction to a magnet.
(1.1ct., Diamagnetic, SI < 0)
The pinkish-brownish-orange Tourmaline pear pictured below (left) also shows no magnetic attraction, as we would expect. The square Tourmaline on the right has similar color, but it shows a Drag response. In fact, it is the most magnetic transparent Tourmaline tested in this study. How is this possible? The causes of color for this gem cannot be definitively determined with a spectrometer, but we can guess that manganese is primarily responsible for the exceptionally strong magnetism. Manganese can contribute to the magnetic susceptibility of Tourmalines of any color without necessarily contributing to color.
Pinkish Brownish Orange
(2.1ct., Drags, SI 547)
Bi-color Tourmaline 2.2ct
Pinkish Brownish Orange
(.62ct., Diamagnetic, SI <0)
(.75ct., Diamagnetic, SI < 0)
"Rubellite" Tourmaline 7.5ct.
Bluish green colors are often seen in Tourmalines. In these instances, color appears to be due to a mixture of iron Fe2+ ions (blue) and iron to titanium (Fe2+-Ti4+) charge transfer (green). Below are 2 examples. The trillion on the left is only moderately magnetic, while the shield on the right shows a Drag response. The difference in magnetic susceptibilities of these 2 gems of similar color is due to a higher concentration of iron in the gem on the right.
(3.65ct. Moderate, SI 65)
(1.6ct, Drags, SI 382)
Blackish Green Tourmaline
(2.69 ct., Strong, SI 130)
In green Tourmalines colored by iron, there is a correlation between color intensity and magnetic susceptibility, just as there is with blue Tourmalines colored by iron. Light green gems show much weaker magnetic responses than dark green "Verdelite" gems (see 3 examples below). We can assume that light green gems are Weak to Moderate due to relatively low iron content, while dark green gems show either a Strong response or a Drag response due to high iron content.
(19.46ct., Strong, SI 295)
Note: Besides helping us to identify or separate gems, a magnetic wand provides a means to quickly and efficiently flag gems with unexpected magnetic properties, such as the blue Tourmalines above. We can isolate these gems as candidates for further analysis in order to uncover what is different about them in terms of causes of color.
Black Tourmaline Rough Crystals 17mm
Tourmaline gems often contain high concentrations of iron and/or manganese, and they are the most magnetic of all natural transparent gemstones that are allochromatic (colored by impurities). For our study of magnetism, we examined over 150 Tourmaline gems in a wide range of colors and shades. The findings presented here include magnetic responses to a magnet as well as magnetic susceptibility measurements taken for each Tourmaline using a Hoover balance.
The rough Tourmaline crystal shown above (right) appears opaque black in reflected light and could easily be mistaken for Schorl. But in transmitted light the rough appears translucent blackish-green. It is the most magnetic of the three Tourmalines above, but still not strong enough to show a Drag response characteristic of dark green "Verdelite" gems and opaque black Schorl. The primary species composition of this crystal could be Elbaite, Dravite or Uvite.
Red Uvite: The Uvite Tourmaline species intermixes with the Dravite species, and the two cannot be individually identified or separated without advanced testing. According to the sellers of the the two red Brazilian Tourmalines shown below, these gems are from deposits known to be Uvite. Both gems are strongly magnetic due to iron (Fe2+).
Brown Tourmaline Rough Crystal
Brownish Red Uvite (1.47ct, Strong, SI 243)
Red Uvite (0.46ct, Strong, SI 218)
Tourmaline species mix with each other in the same way that Garnet species do. Even though Elbaite Tourmaline is considered to be allochromatic, it might be more instructive to describe this gem species as partly allochromatic and partly idiochromatic, as is Grossular Garnet, Pyrope Garnet and even Peridot. As the diagram above shows, Elbaite gems (with the exception of copper-bearing Elbaite) do not derive color and magnetism from impurities as do most allochromatic gems. Elbaite gems derive color and magnetism by mixing with idiochromatic Tourmaline species that contain transition metals in their native chemistry. The blue color in the Elbaite Tourmaline below ("Indicolite" variety) is not caused by impurities of iron, but instead by iron derived from mixing with Schorl Tourmaline.
Blue "Indicolite" Tourmaline is primarily composed of Allochromatic Elbaite Tourmaline Mixed with a Lesser Amount of Idiochromatic Schorl Tourmaline
Cryptic Iron: Iron (Fe3+) is a weak chromophore, and lower concentrations of Fe3+ may not produce color in some gems. These Fe3+ ions can appear invisible or cryptic, as we see in the colorless end of this Indicolite Tourmaline. Such cryptic iron also does not absorb enough light to show detectable absorption lines under a spectroscope, but the iron can be detected with more sensitive instruments such as a spectrometer or a magnetic wand. The spectrometer graph below clearly shows the presence of Fe3+ in completely colorless Tourmaline.
Iron (Fe3+) in a Completely Colorless Section of Tourmaline
Increasing Concentration of Iron in 3 Dark Blue Tourmalines
Spectrometer analysis comparing the 2 gems above to an "Indicolite" gem supports our hypothesis. The graph below shows that the gem above right has a higher iron content than the gem above left, and that both have a lower concentration of iron than "Indicolite" Tourmaline, which shows a drag response.
We can compare relative magnetic susceptibilities of green Tourmalines to blue Tourmalines by referring to the bar graph near the top of this page. The graph shows that pale green gems are on average less magnetic than pale blue gems. Dark green "Verdelite" and dark blue "Indicolite" are equally magnetic.
Information about causes of Tourmaline color was resourced from Dr. George Rossman's CalTech web page on Tourmaline spectroscopy, as well as our own analyses using a spectrometer. The causes of color in Tourmalines can be complex and subtle, and remain only partially understood by researchers.
Brownish Green "Chrome" Tourmaline
Daylight and UV Fluorescence
(0.17ct, Inert, SI <0)
Chrome Tourmaline is a much rarer type of green Tourmaline that can show deep green color. It is colored by low concentrations of chromium ions (Cr3+) derived from mixing with the idiochromatic Chromium-Dravite species, and/or vanadium ions (V3+) derived from mixing with the idiochromatic Vanadium-Dravite species. Despite the trade name "Chrome" Tourmaline, chemical analyses have shown that vanadium is often the primary cause of color. Because of the low concentrations of chromium/vanadium in transparent Chrome Tourmaline, the primary species composition of most gems is Dravite rather than Chromium-Dravite or Vanadium-Dravite. "Chrome" Tourmalines are mostly diamagnetic, but some are faintly attracted to a magnet.
The Chrome Tourmaline shown below has a rich green color typical of the variety, and it is diamagnetic. It glows bright red under a Chelsea filter and fluoresces red under long wave UV light due to chromium. Chrome Tourmaline is the only variety of Tourmaline that we have encountered that fluoresces.
"Chrome" Tourmaline (1.02ct., Inert, SI <0)
Daylight and UV Fluorescence
(1.02ct., Drag, SI 295)
Vanadium Tourmaline (1.57ct, Inert, SI <0)
The two unusual green Tourmalines shown below were sold as Chrome Tourmalines. They show no reaction under a Chelsea filter, and no LWUV fluorescence, suggesting that green color is probably due entirely to vanadium rather than chromium. Strong yellowish green fluorescence under short wave UV light reveals a trace amount of manganese. Spectrometer analysis shows that the light green gem on the right has a lower concentration of vanadium than the moderate green gem on the left. Perhaps a better name for these 2 gems would be Vanadium Tourmaline. Neither gem contains any magnetically detectable iron, and the vanadium and manganese content is also magnetically undetectable. These Vanadium Tourmalines are diamagnetic.
Vanadium Tourmaline (14.10ct, Inert, SI >0)