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 XY
3Z
6(T
6O
18)(BO
3)
3V
3W, where:
[3]
X = Ca, Na, K, vacancy
Y = Li, Mg, Fe
2+, Mn
2+, Zn, Al, Cr
3+, V
3+, Fe
3+, Ti
4+
Z = Mg, Al, Fe
3+, Cr
3+, V
3+
T = Si, Al, B
B = B, vacancy
V = OH, O
W = OH, F, O
Tourmaline belongs to the
trigonal crystal system and occurs as long, slender to thick prismatic and columnar
crystals that are usually triangular in cross-section. Interestingly, the style of termination at the ends of
crystals is 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. Tourmaline crystals when warmed become positively charged at one end and negatively charged at the other. Due to this effect, tourmaline crystals in collections may attract unsightly coatings of dust when displayed under hot spotlights. Tourmaline's unusual electrical properties made it famous in the early 18th century. Brightly colored
Sri Lankan gem tourmalines were brought to Europe in great quantities by the
Dutch East India Company to satisfy a demand for curiosities and gems. At the time it was not realised that 'schorl' and tourmaline were the same mineral.
Tourmaline has a wide variety of
colors. Usually, iron-rich tourmalines are black to bluish-black to deep brown, while magnesium-rich varieties are
brown to
yellow, and lithium-rich tourmalines are practically any color: blue, green, red, yellow, pink etc. Rarely, it is colorless. Bi-colored and multicolored crystals are relatively common, reflecting variations of fluid chemistry during crystallisation. 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. Some forms of tourmaline are
dichroic, in that they appear to change color when viewed from different directions.
The most common variety of tourmaline is 'schorl'. 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 the year
1400 (AD) because a village known today as
Zschorlau (in
Saxony,
Germany) was then named "Schorl" (or minor variants of this name). This village had a nearby tin mine where, in addition to
cassiterite, a lot of black tourmaline was found. The first relatively detailed description of schorl with the name "schürl" and its occurrence (various tin mines in the
Saxony Ore Mountains) was written by
Johannes Mathesius (
1504-
1565) in
1562 under the title "Sarepta oder Bergpostill" (Ertl, 2006). Up to about
1600, additional names used in the
German language were "Schurel", "Schörle", and "Schurl". From the 18th century on, the name "Schörl" was mainly used in the German-speaking area. In
English, the names "shorl" and "shirl" were used in the 18th century for schorl. In the 19th century the names "common schorl", "schörl", "schorl" and "iron tourmaline" were used in the Anglo-Saxon area (Ertl,
2006). The word tourmaline has two entymologies, both from the
Sinhalese word ''turamali'', meaning "stone attracting ash" (a reference to its pyroelectric properties) or according to other sources "mixed gemstones". The meaning of the word "schorl" is a mystery, but it may be a
Scandinavian word.
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 Mineralogieâ€
(published in 1884) for Mg-rich (and Na-rich) tourmaline from the village Unterdrauburg,
Drava river area, Carinthia, Austro-Hungarian Empire. Today this tourmaline locality
(type locality for dravite) at the village Dravograd (near Dobrova pri Dravogradu), is a part of
the Republic of Slovenia (Ertl, 2007). 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 (Ertl, 2007).
Tourmaline is found in two main geological occurrences.
Igneous rocks, in particular
granite and granite
pegmatite 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. Also, tourmaline is a durable mineral and can be found in minor amounts as grains in
sandstone and
conglomerate.
Today, gem and specimen tourmaline is mined chiefly in
Brazil and
Africa. Some placer material suitable for gem use comes from Sri Lanka. Some fine gem and specimen material has been produced in the US, with the first discoveries having been made 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 interesting bicolors. During the early 1900s, Maine and California were the worlds largest producers of gem tourmalines. The Empress Dowager
Tz'u Hsi, the last Empress 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.
[4]
Almost every color of tourmaline can be found in Brazil, especially in the Brazilian states of
Minas Gerais and
Bahia. In
1989, miners discovered a unique and brightly colored variety of tourmaline in the state of ParaÃba. The new type of tourmaline, which soon became known as paraiba tourmaline, came in unusually vivid blues and greens. These colors were often described as "neon" since they appeared to glow. Brazilian paraiba tourmaline is usually heavily included. Much of the paraiba tourmaline from Brazil actually comes from 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. The demand and excitement for this new material, which has fetched more than $50,000 per carat, earned more respect for the other colors of tourmaline. In the late 90s, 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 beautiful tourmaline colored by copper, similar to the Brazilian
paraiba. While its colors are somewhat less bright than top Brazilian material, Mozambique paraiba is often less included and has been found in larger sizes. The Mozambique paraiba material usually is more intensely colored than the Nigerian. There is a significant overlap in color and clarity with Mozambique paraiba and Brazilian paraiba, especially with the material from Rio Grande do Norte. While less expensive than top quality Brazilian paraiba, some Mozambique material sells for well over $5,000 per carat, which still is extremely high compared to other tourmalines.
Tourmaline mineral (~ 10 cm tall)
Another unique variety that is also highly valued is chrome tourmaline, a rare type of dravite tourmaline from
Tanzania which occurs in a very rich green color caused by chromium, the same element which causes the green in emerald. Of the standard Elbaite colors, generally blue indicolite gems are the most expensive, followed by green verdelite and pink to red rubellite. There are also yellow tourmalines, sometimes known as canary tourmaline. Ironically the rarest variety, colorless achroite, is not appreciated and is the least expensive of the transparent tourmalines. In addition to Brazil, significant amounts of tourmaline are also mined in
Tanzania,
Nigeria,
Kenya,
Madagascar,
Mozambique,
Namibia,
Afghanistan,
Pakistan, and
Sri Lanka, and
Malawi.
[5]
Some tourmaline gems, especially pink to red colored stones, are altered by
irradiation to improve their color. Irradiation is almost impossible to detect in tourmalines, and does not impact the value. Heavily-included tourmalines, such as rubellite and Brazilian paraiba are sometimes clarity enhanced, which must be disclosed to the buyer. A clarity-enhanced tourmaline (especially paraiba) is worth much less than a non-treated gem.
[6]
Tourmaline Species and Varieties
Large Pink Elbaite Crystal on Quartz, Cryo-Genie Mine, San Diego Co., California, USA
★ Dravite Species: from the Drave district of
Carinthia)
★
★ Dark yellow to brownish black - dravite
★ Schorl Species:
★
★ Bluish or brownish black to Black - schorl
★ Elbaite Species: named after the island of
Elba,
Italy
★
★ Rose or pink - rubellite variety(from
ruby)
★
★ Dark blue - indicolite variety (from
indigo)
★
★ Light blue -
Brazilian
sapphire variety
★
★ Green - verdelite or Brazilian
emerald variety
★
★ Colorless - achroite variety (from the
Greek for "colorless")
Tri-color Elbaite Crystals on Quartz, Himalaya Mine, San Diego Co., California, USA
References
1. Gemological Institute of America, ''GIA Gem Reference Guide'' 1995, ISBN 0-87311-019-6
2. Mindat tourmaline group Accessed September 12 2005. This website details specifically and clearly how the complicated chemical formula is structured.
3. Hawthorne, F.C. & Henry, D.J. (1999). Classification of the minerals of the tourmaline group. European Journal of Mineralogy, 11, pp 201-215.[1]
4. Exploring and Mining Gems and Gold in the West, By Fred Rynerson - Naturegraph Publishers.
5. Manual of Mineralogy (after Dana), 19th Edition, By Hurlbut and Klien - John Wiley and Sons, Publishers
6. Gemstone Enhancement: Heat, Irradiation, Impregnation, Dyeing, and Other Treatments by Kurt Nassau; Butterworth Publishers 1984
★ Ertl, A., Pertlik, F. & Bernhardt, H.-J. (1997) ''Investigations on olenite with excess boron from the Koralpe, Styria, Austria'', Österreichische Akademie der Wissenschaften, Mathematisch-Naturwissenschaftliche Klasse, Abt. I, Anzeiger, 134, pp 3-10.
Article Online
★ Ertl, A. (2006) ''About the etymology and the type localities of schorl'' Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 152, 2006, pp 7–16.
Article Online
★ Ertl, A. (2007) ''About the type locality and the nomenclature of dravite'' Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 153, 2007, pp 265–271.
Article Online
★ Schumann, Walter (2006) ''Gemstones of the World 3rd Edition'' Sterling Publishing, New York; pp 126-127
External Links
★
Tourmaline classification Accessed
September 12 2005
★
Mindat tourmaline group Accessed
September 12 2005
★
Gemstone.org International Colored Stone Association
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