Most commonly black, but can range from colorless to brown, red, orange, yellow, green, blue, violet, pink, or hues in-between. It can also be bi-colored, or even tri-colored. Rarely, it can be found as neon green or electric blue.
Strong narrow band at 498 nm, and almost complete absorption of red down to 640 nm in blue and green stones; red and pink stones show lines at 458 and 451 nm, as well as a broad band in the green spectrum[1]
Brightly coloredCeylonese gem tourmalines were brought to Europe in great quantities by theDutch 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 itspyroelectric properties.[5]
Tourmalines were used by chemists in the 19th century topolarize light by shining rays onto a cut and polished surface of the gem.[6]
A single stark green fluorite isolated on top of schorl crystalsSchorl, magnified 10×
The most common species of tourmaline isschorl, 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 asZschorlau (inSaxony, Germany) was then named "Schorl" (or minor variants of this name), and the village had a nearbytin mine where, in addition tocassiterite, black tourmaline was found. The first description of schorl with the name "schürl" and its occurrence (various tin mines in theOre Mountains) was written byJohannes Mathesius (1504–1565) in 1562 under the title "Sarepta oder Bergpostill".[7] Up to about 1600, additional names used in theGerman language were "Schurel", "Schörle", and "Schurl". Beginning in the 18th century, the nameSchörl was mainly used in the German-speaking area. In English, the namesshorl andshirl were used in the 18th century. In the 19th century the namescommon schorl,schörl,schorl andiron tourmaline were the English words used for this mineral.[7]
Dravite, also calledbrown 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.[8]
The namedravite was used for the first time byGustav Tschermak (1836–1927), Professor ofMineralogy andPetrography at theUniversity of Vienna, in his bookLehrbuch der Mineralogie (published in 1884) formagnesium-rich (andsodium-rich) tourmaline from village Dobrova nearUnterdrauburg in theDrava river area,Carinthia,Austro-Hungarian Empire. Today this tourmaline locality (type locality for dravite) at Dobrova (nearDravograd), is a part of theRepublic of Slovenia.[9] Tschermak gave this tourmaline the name dravite, for the Drava river area, which is the district along the Drava River (in German:Drau, inLatin:Drave) inAustria and Slovenia. The chemical composition which was given by Tschermak in 1884 for this dravite approximately corresponds to the formulaNaMg3(Al,Mg)6B3Si6O27(OH), which is in good agreement (except for theOH content) with the endmember formula of dravite as known today.[9]
Dravite varieties include the deep green chromium dravite and the vanadium dravite.[10]
Elbaite with quartz and lepidolite on cleavelandite
A lithium-tourmalineelbaite was one of three pegmatitic minerals fromUtö,Sweden, in which the new alkali elementlithium (Li) was determined in 1818 byJohan August Arfwedson for the first time.[11]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 describedfluorine (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 fromSušice,Czech Republic. In 1914,Vladimir Vernadsky proposed the nameElbait for lithium-, sodium-, and aluminum-rich tourmaline from Elba Island, Italy, with the simplified formula(Li,Na)HAl6B2Si4O21.[11] 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.[11] In 1933 Winchell published an updated formula for elbaite,H8Na2Li3Al3B6Al12Si12O62, which is commonly used to date written asNa(Li1.5Al1.5)Al6(BO3)3[Si6O18](OH)3(OH).[11] The first crystal structure determination of a Li-rich tourmaline was published in 1972 by Donnay and Barton, performed on a pink elbaite fromSan Diego County,California, United States.[citation needed]
The tourmaline mineral group is chemically one of the most complicated groups ofsilicate minerals. Its composition varies widely because ofisomorphous replacement (solid solution), and its general formula can be written asXY3Z6(T6O18)(BO3)3V3W, where:[12]
Tri-chromatic elbaite crystals on quartz, Himalaya Mine, San Diego Co., California, US
Tourmaline is a six-member ringcyclosilicate having atrigonal crystal system. It occurs as long, slender to thick prismatic and columnarcrystals 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-grainedgranite calledaplite, 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 perfectlyeuhedral. An exception was the fine dravite tourmalines ofYinnietharra, in western Australia. The deposit was discovered in the 1970s, but is now exhausted. All hemimorphic crystals arepiezoelectric, and are oftenpyroelectric as well.[citation needed]
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 ofkaolin. 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 aunit cell: The actual unit cell of this structure includes portions of several units belonging to adjacent columns.[17][18]
Oblique view of a single unit of the tourmaline crystal structure.
View of single unit of tourmaline structure along the axis of the crystal
View along a axis of three columns of tourmaline units forming a bundle
Structure of a tourmaline crystal viewed looking along the c axis of the crystal
Two dark-green rectangular tourmaline stones and one oval tourmaline stoneBi-chromatic tourmaline crystal, 0.8 inches (2 cm) longTourmaline mineral, approximately 10 cm (3.9 in) tall
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 calledwatermelon tourmaline and is prized in jewelry. An excellent example of watermelon tourmaline jewelry is a brooch piece (1969, gold, watermelon tourmaline, diamonds) byAndrew Grima (British, b. Italy, 1921–2007), in the collection of Kimberly Klosterman and on display at theCincinnati Art Museum.[19] Some forms of tourmaline aredichroic; they change color when viewed from different directions.[20]
The pink color of tourmalines from many localities is the result of prolonged natural irradiation. During their growth, these tourmaline crystals incorporatedMn2+ and were initially very pale. Due to naturalgamma ray exposure fromradioactive decay of40K in theirgranitic environment, gradual formation of Mn3+ ions occurs, which is responsible for the deepening of the pink to red color.[21]
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).[citation needed]
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 handheldneodymium 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.[22]
Some tourmaline gems, especially pink to red colored stones, are altered byheat 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.[23]
Tourmaline is found ingranite and granitepegmatites and inmetamorphic rocks such asschist andmarble. 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 insandstone andconglomerate, and is part of theZTR index for highly weathered sediments.[24]
Some fine gems and specimen material have been produced in the United States, with the first discoveries in 1822, in the state ofMaine. 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. TheEmpress Dowager Cixi of China loved pink tourmaline and bought large quantities for gemstones and carvings from the then new Himalaya Mine, located inSan Diego County, California.[26] 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 atPala, California inSan Diego County.[27]
Watermelon Tourmaline mineral on quartz matrix (crystal approximately 2 cm (0.79 in) wide at face)
Almost every color of tourmaline can be found in Brazil, especially inMinas Gerais andBahia. 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 fromParaíba, but the neighboring state ofRio 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.[28]
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.[29][30] Owned by Billionaire Business Enterprises,[29] it was presented inMontreal,Quebec,Canada, on 14 October 2009.[30]
In the late 1990s, copper-containing tourmaline was found inNigeria. 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 fromMozambique 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.[31]
Another highly valuable variety is chrome tourmaline, a rare type of dravite tourmaline fromTanzania. 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,[32] followed by green verdelite and pink to red rubellite.[33]
^"Tourmaline".Mindat.org. Archived fromthe original on 2005-12-28. Retrieved2005-09-12. This website details specifically and clearly how the complicated chemical formula is structured.{{cite web}}: CS1 maint: postscript (link)
^Erratum: American Mineralogist (2013), Volume 98, page 524.
^Frank C. Hawthorne and Dona M. Dirlam. "Tourmaline: Tourmaline the Indicator Mineral: From Atomic Arrangement to Viking Navigation."Elements, October 2011, v. 7, p. (5): 307–312, doi:10.2113/gselements.7.5.307.
^Hamburger, Gabrielle E.; Buerger, M.J. (1948)."The structure of tourmaline".American Mineralogist.33 (9–10):532–540. Retrieved15 February 2021.
^Nesse, William D. (2000).Introduction to mineralogy. New York: Oxford University Press. pp. 303–304.ISBN9780195106916.
