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.
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.
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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.
Tourmaline Species Mix Together in Solid Solution Series.
The Chromophores for 3 Idiochromatic Species are Highlighted in Red.
Contents of this Section
P.1) Overview: Tourmaline SpeciesP.2) Black, Blue and Green Tourmalines
Color and Magnetism
Overview: Tourmaline Species
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.
Bi-color Tourmaline 2.2ct
"Rubellite" Tourmaline 7.5ct.
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.
Uvite: The Uvite Tourmaline species is often included and rarely faceted as gemstones. Uvite 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+).
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
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.
Uvite Tourmaline mixes with the Vanadium-Dravite and Chromium-Dravite species. Green Uvite is usually full of inclusions and rarely faceted into gemstones. According to the seller of the green Tourmaline rough crystal shown below, this is green Uvite from Burma.
Green Uvite Crystal
Using spectroscopy, fluorescence, Chelsea filter and magnetic testing, it appears that the coloring agent in this green Tourmaline crystal is primarily vanadium, with some chromium. This Uvite above does not fluoresce and is diamagnetic.