Tourmaline

Tourmaline is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a wide variety of colors.

The name is derived from the Sinhalese tōramalli (ටෝරමල්ලි), which refers to the carnelian gemstones.

History

Brightly colored Ceylonese gem tourmalines were brought to Europe in great quantities by the Dutch East India Company to satisfy a demand for curiosities and gems. Tourmaline was sometimes called the "Ceylonese Magnet" because it could attract and then repel hot ashes due to its pyroelectric properties.

Tourmalines were used by chemists in the 19th century to polarize light by shining rays onto a cut and polished surface of the gem.

Ancient traditions of miraculous powers

The first description of a mineral with the properties of tourmaline, at least in Europe, was given by Theophrastus of Eresus (371 – 287 BC) in his work De lapidibus. He describes lyngurium as a clear, hard and cold-to-the-touch gemstone that has the ability to attract other materials such as straw and leaves or even thin flakes of copper or iron. Tradition has it that it is formed from the urine of lynxes. They cover their urine with earth, so it can only be discovered by very experienced collectors. These were apparently rare, and Pliny the Elder reported in his Naturalis historia around 77 AD that lynxes held a grudge against humans and that probably no one in his time had ever seen this stone, for which he introduced the Latin name lyncurium. He considers all stories about lyncurium to be false. Rather mythological descriptions of lingurium can nevertheless be found in numerous works on gemstones up to the Middle Ages.  

Persian gemologists and the fascination of colors

A gemstone with a color combination of red, yellow, or green in a single crystal, particularly that of tourmaline, has probably been known in the Persian-Arabic region since the 9th century. The Persian polymath al-Bīrūnī, who partly relies on works by al-Kindī and ad-Dīnawarī from the 9th and 10th centuries, wrote in his General Gemstone Studies in the 11th century about the gemstone La'l: "...it is very often spoken of a piece of La'l that is partly red, partly yellow. Some gemologists mention a La'l that is red, yellow, and green, not as a distinction between the different types, but as a union of colors in one piece. " A more comprehensive description of the La'l, together with a description of an occurrence in a geode, is given by Muhammad Ibn Mansur in 1491 in his "Gawahirnama - Collected Knowledge about Gemstones." 

Medieval miners and the discovery of Schorl

Schorl is the first mineral from the tourmaline group to be described as such in European literature. It occurs together with tin ore in the river sediments of the Ore Mountains, which have been mined since the 12th century by immigrant miners from the Fichtel Mountains. The name Schorl was probably in use in various spellings before 1400, but was first recorded as Schörlein in 1505 by Rülein of Calw in his " well-ordered and useful little book on how to seek and find a mine." 

Nearly 60 years later, in 1562, the German pastor Johannes Mathesius published his Sarepta Oder Bergpostill, Sampt der Joachimßthalischen kurtzen Chroniken (Sarepta or Bergpostill, including the Joachimsthal short chronicles), a collection of 16 sermons. In the 9th sermon, "On Tin / Lead / Glet / Bismuth and Spießglaß" (On Tin / Lead / Glet / Bismuth and Spießglaß), written in 1559, he mentions Schürl (Scurrile), which occurs together with tin ore and should not be smelted together with it.  

Tourmalines were already popular gemstones in the Middle Ages, even though they were not yet distinguished from other gemstones such as ruby, beryl, or garnet. For example, a central "ruby" in the Crown of St. Wenceslas, which was made for Emperor Charles IV (Holy See) in the 14th century, is a red tourmaline. 

Dutch imports and the discovery of pyroelectricity

The name tourmaline was used in Europe from ~1700 and comes from the Sinhalese word thuramali (තුරමලි) or thoramalli (තෝරමල්ලි). The German physician and botanist Paul Hermann was probably the first to bring gemstones with this name to Europe. From 1672 to 1677 he traveled to Ceylon as a doctor for the Dutch East India Company, where he amassed an extensive collection of natural history specimens. The collection was auctioned after his death (1695) and the catalog was printed in 1711. It also includes numerous gemstones and under number 197 a "Chrysolithos Turmale Zeyl." (Zeylanicus).  In Ceylon, various stones were called turemali. As the Swedish naturalist and physician Carl Peter Thunberg reported in 1784 in his "Description of the Minerals and Gems of the Island of Ceylon," these included bluish quartz (Nile turemali), chrysolite with a quadrilateral prism (Patje turemali), greenish-yellow topaz (Kaneke turemali), or white-yellow topaz (Sudu turemali). Schorl was known in Ceylon as Kallu Palingu (black crystal). 

Among these colorful gemstones, some stood out for their property, which Theophrastus had described around 2000 years earlier. In 1707, Johann Georg Schmidt, in his Curious Speculations on Sleepless Nights, recounted the report of Dr. Daumius, staff physician of the Royal Polish and Electoral Saxon militia on the Rhine. He had told him that in 1703 the Dutch had imported a gemstone, tourmaline or tourmales, from Ceylon, which, when heated, could attract ash and was therefore also called an ashentrekker. Ten years later, the physicist and chemist Louis Lémery presented a tourmaline with this property to the Académie des sciences in Paris. In 1744, the Danish pharmacist August Günther Carl von Linné asked him to help him identify the plants from the herbaria that Paul Hermann had compiled in Ceylon. Linnaeus published his results in 1747 in his Flora Zeylanica, in the preface of which he also describes a lapidem electricum (electric stone) – even before Franz Ulrich Theodor Aepinus was able to prove the electrical nature of tourmaline's attraction in 1756. Apenius described the electrical charge at the ends of a tourmaline crystal as a result of heating and was the first to observe that the crystal ends become oppositely charged, positively and negatively. David Brewster introduced the term pyroelectricity in 1824. 

Cataloging Diversity: Exploring Composition

The second half of the 18th century saw a slight inflation of new mineral descriptions with the name schorl. Many minerals that could not be identified were considered new varieties of schorl. René-Just Haüy lists 16 different schorls with a wide range of properties and considers the name so compromised that he wanted to remove it completely from mineralogical nomenclature. Martin Heinrich Klaproth does not go that far, but emphasizes the central importance of chemical analysis for mineral identification. Torbern Olof Bergman conducted the first attempts in this area as early as 1779. He found clay (Al2O3), silica (SiO2), lime (CaO), and iron (FeO). Johann Christian Wiegleb published the first analysis of schorl in 1785, and in 1798 Wondraschek in Prague also detected manganese dioxide (MnO) and water in a reddish tourmaline from Moravia.  

All of these early analyses lack the element boron, an essential component of all tourmalines, which was only discovered in 1808 by Joseph Louis Gay-Lussac and Louis Jacques Thénard.  Finally, in 1818 in Munich, A. Vogel, following suggestions from August Breithaupt and Christian Gottlob Gmelin, succeeded in detecting the previously overlooked element boron in tourmaline. In the same year, Johan August Arfwedson published his analyses of minerals from the island of Utö in Sweden. In the mineral petalite, he discovered the element lithium, which he was also able to detect in a tourmaline from the iron ore deposit. In 1850, Carl Rammelsberg was able to add fluorine to the list of elements in tourmaline. In the mid-19th century, 12 elements were known from numerous tourmaline analyses (H, Li, Na, K, Ca, Mg, Fe, Mn, B, Al, Si, F), but a general formula for tourmalines was not recognized. John Ruskin commented in 1866 that the chemistry of tourmaline was more like a medieval doctor's prescription than a proper mineral composition. 

By 2018, the number of elements (including vacancies) detected in tourmalines with significant concentrations had grown to around 26. 

Order of Diversity: Exploring Structure

Jean-Baptiste Romé de L'Isle, one of the founders of crystallography, conducted systematic studies of the crystal forms of numerous minerals. In 1772, he noticed the close relationship between schorl, the transparent tourmalines from Ceylon, and some gemstones that came to Europe from Brazil. 

The decisive turning point in the study of crystals came at the beginning of the 20th century, when Max von Laue described the diffraction of X-rays by crystal lattices in 1912, making it possible for the first time not only to determine the symmetry of a crystal, but also to elucidate its structure, the arrangement of atoms within the crystal. Charlotte Kulaszewski took the first Laue images of tourmaline in 1921 in Leipzig and described the observed X-ray diffraction patterns as having a hexagonal symmetry. 

The hexagonal description of the X-ray diffraction patterns contradicted the trigonal symmetry of the crystal forms of tourmaline, which prompted Martin J. Buerger and William Parrish, at the suggestion of Joseph D.H. Donnay, to redetermine the symmetry. In 1937, using the Weissenberg method, they succeeded in determining the correct trigonal symmetry of tourmaline with the space group R 3 m (space group no. 160). 

Based on this symmetry, 11 years later, Gabrielle E. Hamburger and Martin J. Buerger at the Massachusetts Institute of Technology succeeded in determining the structure of tourmaline. They described tourmaline as a ring silicate with 5 different lattice positions, which are surrounded 3-fold, 4-fold or 6-fold by anions (O 2−, OH −, F −) at 8 different positions. Starting from this structure, they were able to determine the structural formula of colorless Mg-Al tourmaline with NaMg3 B3 Al6 Si6 O27 (OH)4 and thus laid the basis for the definition of the various minerals of the tourmaline group. 

When the Commission on New Minerals and Mineral Names (CNMMN) of the International Mineralogical Association was founded in 1959, only four minerals were distinguished in the tourmaline group: schorl, dravite, elbaite and uvite. By 1997, when Frank C. Hawthorne and Darrell J. Henry presented their still unofficial interim tourmaline classification at the International Tourmaline Conference in the Czech Republic (Tourmaline 97 meeting), the tourmaline group had already grown to 12 recognized minerals and 27 hypothetical end members in three subgroups. The current classification of the tourmaline supergroup, recognized by the IMA-CNMNC, had already grown to 18 recognized minerals and 22 hypothetical end members in three groups with a total of 14 subgroups at the time of its publication in 2011. Currently (2023) 40 minerals are listed in the tourmaline group.

Classification

The supergroup of tourmalines is divided into primary groups and secondary subgroups. The occupation of the X-position with alkali ions (Na, K), calcium, or vacancies is the criterion for the three primary tourmaline groups:

Alkali group: (Na + + K +) > Ca 2+ and (Na + + K +) > □

Calcium group: Ca 2+ > (Na + + K +) and Ca 2+ > □

X-vacancy group: □ > (Na + + K +) and □ > Ca 2+

The occupancy schemes and coupled substitutions at positions Y, Z, V, and W provide the criteria for the further subgroups of the primary tourmaline groups. 

Species and varieties

Commonly encountered species and varieties of tourmaline include the following:

Schorl species

Brownish-black to black—schorl

Dravite species (from the Drave district of Carinthia)

Dark yellow to brownish-black—dravite

Elbaite species (named after the island of Elba, Italy)

Red or pinkish-red—rubellite variety

Light blue to bluish-green—indicolite variety (from indigo)

Green—verdelite variety

Colorless—achroite variety (from Ancient Greek άχρωμος (ákhrōmos) 'colorless')

Schorl

The most common species of tourmaline is schorl, the sodium iron (divalent) endmember of the group. It may account for 95% or more of all tourmaline in nature. The early history of the mineral schorl shows that the name "schorl" was in use prior to 1400 because a village known today as Zschorlau (in Saxony, Germany) was then named "Schorl" (or minor variants of this name), and the village had a nearby tin mine where, in addition to cassiterite, black tourmaline was found. The first description of schorl with the name "schürl" and its occurrence (various tin mines in the Ore Mountains) was written by Johannes Mathesius (1504–1565) in 1562 under the title "Sarepta oder Bergpostill". Up to about 1600, additional names used in the German language were "Schurel", "Schörle", and "Schurl". Beginning in the 18th century, the name Schörl was mainly used in the German-speaking area. In English, the names shorl and shirl were used in the 18th century. In the 19th century the names common schorl, schörl, schorl and iron tourmaline were the English words used for this mineral.

Dravite

Dravite, also called brown tourmaline, is the sodium magnesium rich tourmaline endmember. Uvite, in comparison, is a calcium magnesium tourmaline. Dravite forms multiple series, with other tourmaline members, including schorl and elbaite.

The name dravite was used for the first time by Gustav Tschermak (1836–1927), Professor of Mineralogy and Petrography at the University of Vienna, in his book Lehrbuch der Mineralogie (published in 1884) for magnesium-rich (and sodium-rich) tourmaline from village Dobrova near Unterdrauburg in the Drava river area, Carinthia, Austro-Hungarian Empire. Today this tourmaline locality (type locality for dravite) at Dobrova (near Dravograd), is a part of the Republic of Slovenia. Tschermak gave this tourmaline the name dravite, for the Drava river area, which is the district along the Drava River (in German: Drau, in Latin: Drave) in Austria and Slovenia. The chemical composition which was given by Tschermak in 1884 for this dravite approximately corresponds to the formula NaMg3(Al,Mg)6B3Si6O27(OH), which is in good agreement (except for the OH content) with the endmember formula of dravite as known today.

Dravite varieties include the deep green chromium dravite and the vanadium dravite.

Elbaite

A lithium-tourmaline elbaite was one of three pegmatitic minerals from Utö, Sweden, in which the new alkali element lithium (Li) was determined in 1818 by Johan August Arfwedson for the first time. Elba Island, Italy, was one of the first localities where colored and colorless Li-tourmalines were extensively chemically analysed. In 1850, Karl Friedrich August Rammelsberg described fluorine (F) in tourmaline for the first time. In 1870, he proved that all varieties of tourmaline contain chemically bound water. In 1889, Scharitzer proposed the substitution of (OH) by F in red Li-tourmaline from Sušice, Czech Republic. In 1914, Vladimir Vernadsky proposed the name Elbait for lithium-, sodium-, and aluminum-rich tourmaline from Elba Island, Italy, with the simplified formula (Li,Na)HAl6B2Si4O21. Most likely the type material for elbaite was found at Fonte del Prete, San Piero in Campo, Campo nell'Elba, Elba Island, Province of Livorno, Tuscany, Italy. In 1933 Winchell published an updated formula for elbaite, H8Na2Li3Al3B6Al12Si12O62, which is commonly used to date written as Na(Li1.5Al1.5)Al6(BO3)3(OH)3(OH). The first crystal structure determination of a Li-rich tourmaline was published in 1972 by Donnay and Barton, performed on a pink elbaite from San Diego County, California, United States.

Chemical composition

The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. Its composition varies widely because of isomorphous replacement (solid solution), and its general formula can be written as XY3Z6(T6O18)(BO3)3V3W, where:

X = Ca, Na, K, ▢ = vacancy

Y = Li, Mg, Fe2+, Mn2+, Zn, Al, Cr3+, V3+, Fe3+, Ti4+, ▢ = vacancy

Z = Mg, Al, Fe3+, Cr3+, V3+

T = Si, Al, B

B = B, ▢ = vacancy

V = OH, O

W = OH, F, O

The 41 minerals in the group (endmember formulas) recognized by the International Mineralogical Association

Species Name	Ideal Endmember Formula	IMA Number	Symbol
Adachiite	CaFe2+3Al6(Si5AlO18)(BO3)3(OH)3OH	2012-101	Adc
Alumino-oxy-rossmanite	▢Al3Al6(Si5AlO18)(BO3)3(OH)3O	2020-008	Aorsm
Bosiite	NaFe3+3(Al4Mg2)Si6O18(BO3)3(OH)3O	2014-094	Bos
Celleriite	▢(Mn2+2Al)Al6(Si6O18)(BO3)3(OH)3(OH)	2019-089	Cll
Chromium-dravite	NaMg3Cr6Si6O18(BO3)3(OH)3OH	1982-055	Cdrv
Chromo-alumino-povondraite	NaCr3(Al4Mg2)Si6O18(BO3)3(OH)3O	2013-089	Capov
Darrellhenryite	NaLiAl2Al6Si6O18(BO3)3(OH)3O	2012-026	Dhry
Dravite	NaMg3Al6Si6O18(BO3)3(OH)3OH	- 1884 -	Drv
Dutrowite	Na(Fe2.5Ti0.5)Al6Si6O18(BO3)3(OH)3O	2019-082	Dtw
Elbaite	Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3OH	- 1913 -	Elb
Ertlite	NaAl3Al6(Si4B2O18)(BO3)3(OH)3O	2023-086	Etl
Ferro-bosiite	NaFe3+3(Al4Fe2+2)Si6O18(BO3)3(OH)3O	2022-069	Fbos
Feruvite	CaFe2+3(MgAl5)Si6O18(BO3)3(OH)3OH	1987-057	Fer
Fluor-buergerite	NaFe3+3Al6Si6O18(BO3)3O3F	1965-005	Fbu
Fluor-dravite	NaMg3Al6Si6O18(BO3)3(OH)3F	2009-089	Fdrv
Fluor-elbaite	Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3F	2011-071	Felb
Fluor-liddicoatite	Ca(Li2,Al)Al6Si6O18(BO3)3(OH)3F	1976-041	Fld
Fluor-rossmanite	▢(LiAl2)Al6Si6O18(BO3)3(OH)3F	2023-111	Frsm
Fluor-schorl	NaFe2+3Al6Si6O18(BO3)3(OH)3F	2010-067	Fsrl
Fluor-tsilaisite	NaMn2+3Al6Si6O18(BO3)3(OH)3F	2012-044	Ftl
Fluor-uvite	CaMg3(Al5Mg)Si6O18(BO3)3(OH)3F	- 1930 -	Fluvt
Foitite	▢(Fe2+2Al)Al6Si6O18(BO3)3(OH)3OH	1992-034	Foi
Lucchesiite	Ca(Fe2+)3Al6Si6O18(BO3)3(OH)3O	2015-043	Lcc
Magnesio-dutrowite	Na(Mg2.5Ti0.5)Al6Si6O18(BO3)3(OH)3O	2023-015	Mdtw
Magnesio-foitite	▢(Mg2Al)Al6Si6O18(BO3)3(OH)3OH	1998-037	Mfoi
Magnesio-lucchesite	Ca(Mg3Al6Si6O18(BO3)3(OH)3O	2019-025	Mlcc
Maruyamaite	K(MgAl2)(Al5Mg)Si6O18(BO3)3(OH)3O	2013-123	Mry
Olenite	NaAl3Al6Si6O18(BO3)3O3OH	1985-006	Ole
Oxy-chromium-dravite	NaCr3(Mg2Cr4)Si6O18(BO3)3(OH)3O	2011-097	Ocdrv
Oxy-dravite	Na(Al2Mg)(Al5Mg)Si6O18(BO3)3(OH)3O	2012-004	Odrv
Oxy-foitite	▢(Fe2+Al2)Al6Si6O18(BO3)3(OH)3O	2016-069	Ofoi
Oxy-schorl	Na(Fe2+2Al)Al6Si6O18(BO3)3(OH)3O	2011-011	Osrl
Oxy-vanadium-dravite	NaV3(V4Mg2)Si6O18(BO3)3(OH)3O	1999-050	Ovdrv
Povondraite	NaFe3+3(Fe3+4Mg2)Si6O18(BO3)3(OH)3O	1979	Pov
Princivalleite	Na(Mn2Al)Al6Si6O18(BO3)3(OH)3O	2020-056	Pva
Rossmanite	▢(LiAl2)Al6Si6O18(BO3)3(OH)3OH	1996-018	Rsm
Schorl	NaFe2+3Al6Si6O18(BO3)3(OH)3OH	- 1505 -	Srl
Tsilaisite	NaMn2+3Al6Si6O18(BO3)3(OH)3OH	2011-047	Tsl
Uvite	CaMg3(Al5Mg)Si6O18(BO3)3(OH)3OH	2000-030	Uvt
Vanadio-oxy-chromium-dravite	NaV3(Cr4Mg2)Si6O18(BO3)3(OH)3O	2012-034	Vocdrv
Vanadio-oxy-dravite	NaV3(Al4Mg2)Si6O18(BO3)3(OH)3O	2012-074	Vodrv

Mineral species that were named before the IMA was founded in 1958 do not have an IMA number.
The IMA commission on new mineral names published a list of approved symbols for each mineral species in 2021.

A revised nomenclature for the tourmaline group was published in 2011.

Physical properties

Crystal structure

Tourmaline is a six-member ring cyclosilicate having a trigonal crystal system. It occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section, often with curved striated faces. The style of termination at the ends of crystals is sometimes asymmetrical, called hemimorphism. Small slender prismatic crystals are common in a fine-grained granite called aplite, often forming radial daisy-like patterns. Tourmaline is distinguished by its three-sided prisms; no other common mineral has three sides. Prisms faces often have heavy vertical striations that produce a rounded triangular effect. Tourmaline is rarely perfectly euhedral. An exception was the fine dravite tourmalines of Yinnietharra, in western Australia. The deposit was discovered in the 1970s, but is now exhausted. All hemimorphic crystals are piezoelectric, and are often pyroelectric as well.

A crystal of tourmaline is built up of units consisting of a six-member silica ring that binds above to a large cation, such as sodium. The ring binds below to a layer of metal ions and hydroxyls or halogens, which structurally resembles a fragment of kaolin. This in turn binds to three triangular borate ions. Units joined end to end form columns running the length of the crystal. Each column binds with two other columns offset one-third and two-thirds of the vertical length of a single unit to form bundles of three columns. Bundles are packed together to form the final crystal structure. Because the neighboring columns are offset, the basic structural unit is not a unit cell: The actual unit cell of this structure includes portions of several units belonging to adjacent columns.

Silicate anion

The cations at the T-position (Si4+, Al3+, B3+) are connected by four oxygen atoms in such a way that the oxygen atoms lie on the vertices of a tetrahedron, with the cation at its center. Tourmalines are ring silicates. Their TO4 tetrahedra are connected via two vertices to neighboring TO4 tetrahedra to form unbranched 6-membered single rings of the composition [Si6 O18 ] −12. 

Borate anion

The cations at the B position (boron) are surrounded by three oxygen atoms. All atoms in the ion −3 lie in a plane. The oxygen atoms are located at the corners of a triangle, with the boron cation at its center. 

X-position

The cations at the X positions are surrounded by nine to ten oxygen atoms. The oxygen atoms lie on the corners of a trigonal antiprism, with the singly to doubly charged cations at the center. 

Y-position

The mostly divalent cations at the Y position are surrounded by six oxygen atoms in an octahedral arrangement. The oxygen atoms lie at the corners of an octahedron, with the cation at its center. Three of these octahedra are connected to each other via shared edges to form trimers. 

Z position

The mostly trivalent cations (Al,…) at the Z position are also octahedrally surrounded by six oxygen atoms. 

Overall structure

The 6-membered silicate rings, M 2+ octahedral trimers (Y-position), and trigonal antiprism of the X-position are stacked on top of each other along the polar z-axis. The free tetrahedral vertices of the silicate rings all point opposite the z-axis and are connected to vertices of the M 2+ octahedral trimers. The cations at the X-position are located centrally above the silicate rings and connect them to the overlying M 2+ octahedral trimer. The M 3+ octahedra of the Z-position are linked via common edges to form chains along the z-axis and connect adjacent stacks of X-, Y-, and tetrahedral positions.

The planar BO 3 anions lie in the ab plane and link the X coordination polyhedra with Z octahedra. 

Color

Tourmaline has a variety of colors. Iron-rich tourmalines are usually black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are almost any color: blue, green, red, yellow, pink, etc. Rarely, it is colorless. Bi-colored and multicolored crystals are common, reflecting variations of fluid chemistry during crystallization. Crystals may be green at one end and pink at the other, or green on the outside and pink inside; this type is called watermelon tourmaline and is prized in jewelry. An excellent example of watermelon tourmaline jewelry is a brooch piece (1969, gold, watermelon tourmaline, diamonds) by Andrew Grima (British, b. Italy, 1921–2007), in the collection of Kimberly Klosterman and on display at the Cincinnati Art Museum. Some forms of tourmaline are dichroic; they change color when viewed from different directions.

The pink color of tourmalines from many localities is the result of prolonged natural irradiation. During their growth, these tourmaline crystals incorporated Mn2+ and were initially very pale. Due to natural gamma ray exposure from radioactive decay of 40K in their granitic environment, gradual formation of Mn3+ ions occurs, which is responsible for the deepening of the pink to red color.

Magnetism

Opaque black schorl and yellow tsilaisite are idiochromatic tourmaline species that have high magnetic susceptibilities due to high concentrations of iron and manganese respectively. Most gem-quality tourmalines are of the elbaite species. Elbaite tourmalines are allochromatic, deriving most of their color and magnetic susceptibility from schorl (which imparts iron) and tsilaisite (which imparts manganese).

Red and pink tourmalines have the lowest magnetic susceptibilities among the elbaites, while tourmalines with bright yellow, green and blue colors are the most magnetic elbaites. Dravite species such as green chromium dravite and brown dravite are diamagnetic. A handheld neodymium magnet can be used to identify or separate some types of tourmaline gems from others. For example, blue indicolite tourmaline is the only blue gemstone of any kind that will show a drag response when a neodymium magnet is applied. Any blue tourmaline that is diamagnetic can be identified as paraiba tourmaline colored by copper in contrast to magnetic blue tourmaline colored by iron.

Treatments

Some tourmaline gems, especially pink to red colored stones, are altered by heat treatment to improve their color. Overly dark red stones can be lightened by careful heat treatment. The pink color in manganese-containing near-colorless to pale pink stones can be greatly increased by irradiation with gamma-rays or electron beams. Irradiation is almost impossible to detect in tourmalines, and does not, currently, affect the value. Heavily included tourmalines, such as rubellite and Brazilian paraiba, are sometimes clarity-enhanced. A clarity-enhanced tourmaline (especially the paraiba variety) is worth much less than an untreated gem of equal clarity.

Use

Particularly beautiful examples are used as gemstones, such as rubellite, a red variant of tourmaline. The most famous example is probably the Bundesliga championship trophy, which is set with a total of 21 tourmalines. The German Football Association (DFB) Cup also features tourmalines.

Due to its effect as a polarizing filter, polished tourmaline discs were used in photography as early as the 19th century to suppress annoying glare. Tourmaline polarizing filters, along with those made of calcite and herapathite, also found their way into microscopy early on, and polarizing microscopes were developed from them. Due to its special electrical properties, tourmaline is also used in electronics.

Geology

Tourmaline is found in granite and granite pegmatites and in metamorphic rocks such as schist and marble. Schorl and lithium-rich tourmalines are usually found in granite and granite pegmatite. Magnesium-rich tourmalines, dravites, are generally restricted to schists and marble. Tourmaline is a durable mineral and can be found in minor amounts as grains in sandstone and conglomerate, and is part of the ZTR index for highly weathered sediments.

Localities

Gem and specimen tourmaline is mined chiefly in Brazil and many parts of Africa, including Tanzania, Nigeria, Kenya, Madagascar, Mozambique, Malawi, and Namibia. It is also mined in Asia, notably in Pakistan, Afghanistan, and Indonesia as well as in Sri Lanka and India, where some placer material suitable for gem use is found.

United States

Some fine gems and specimen material have been produced in the United States, with the first discoveries in 1822, in the state of Maine. California became a large producer of tourmaline in the early 1900s. The Maine deposits tend to produce crystals in raspberry pink-red as well as minty greens. The California deposits are known for bright pinks, as well as bicolors. During the early 1900s, Maine and California were the world's largest producers of gem tourmalines. The Empress Dowager Cixi of China loved pink tourmaline and bought large quantities for gemstones and carvings from the then new Himalaya Mine, located in San Diego County, California. It is not clear when the first tourmaline was found in California. Native Americans have used pink and green tourmaline as funeral gifts for centuries. The first documented case was in 1890 when Charles Russel Orcutt found pink tourmaline at what later became the Stewart Mine at Pala, California in San Diego County.

Brazil

Almost every color of tourmaline can be found in Brazil, especially in Minas Gerais and Bahia. The new type of tourmaline, which soon became known as paraiba tourmaline, came in blue and green. Brazilian paraiba tourmaline usually contains abundant inclusions. Much of the paraiba tourmaline from Brazil does not actually come from Paraíba, but the neighboring state of Rio Grande do Norte. Material from Rio Grande do Norte is often somewhat less intense in color, but many fine gems are found there. It was determined that the element copper was important in the coloration of the stone.

A large bluish-green tourmaline from Paraiba, measuring 36.44 mm × 33.75 mm × 21.85 mm (1.43 in × 1.33 in × 0.86 in) and weighing 191.87 carats (1.3536 oz; 38.374 g), is the world's largest cut tourmaline. Owned by Billionaire Business Enterprises, it was presented in Montreal, Quebec, Canada, on 14 October 2009.

Africa

In the late 1990s, copper-containing tourmaline was found in Nigeria. The material was generally paler and less saturated than the Brazilian materials, although the material generally was much less included. A more recent African discovery from Mozambique has also produced tourmaline colored by copper, similar to the Brazilian paraiba. The Mozambique paraiba material usually is more intensely colored than the Nigerian and Mozambique Paraiba tourmaline have similar colors to the Brazilian Paraiba, but the prices are relatively cheaper, better clarity and larger sizes. In recent years the pricing of these beautiful gemstones has increased significantly.

Another highly valuable variety is chrome tourmaline, a rare type of dravite tourmaline from Tanzania. Chrome tourmaline is a rich green color due to the presence of chromium atoms in the crystal. Of the standard elbaite colors, blue indicolite gems are typically the most valuable, followed by green verdelite and pink to red rubellite.

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