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: The Tourmaline Group



Tourmaline Gemstones used in Jewelry


Rough & Cut Tourmaline Crystals


Source: Afghanistan, Africa, Brazil, Madagascar, Mozambique, Namibia, Nigeria, Sri Lanka, Tanzania, USA (California, Maine), Zambia

Birthstone: October (Alternate: Opal)


Article Copyright © 2008 AllAboutGemstones.com

The name "tourmaline" (tourmalin German, turmalina Spanish, tormalina Italian) is derived from the Sinhalese (Sri Lankan) word "tura mali," meaning "stone of mixed colors." Tourmaline occurs in a wide range of colors which include blue, colorless, green, pink, yellow, red, and black; and encompasses a range of individual minerals that make up the so-called "tourmaline group."



The mercurial nature of tourmailine's color(s) can be very dynamic, in that it can have a very different color and/or saturation depending on which direction the stone is viewed (anisotropism). It is for this reason that the lapidary must reconcile yield vs appearance in order to get the most of the rough stone. See: Cutting & Faceting of Tourmaline for more info.

Although tourmaline occurs in all colors of the spectrum, and in many combinations of colors, pink and green are the two most common colors used as gemstones. Many tourmaline specimens have a more saturated color down the c-axis, or "principle axis," with a different shade of the same color, or a completely different color running perpendicularly to the c-axis, along the a/b axises. The color of the a-axis and the b-axis must be the same because of the geometry of the crystallization of tourmaline.


Tourmaline Nomenclature

As the nomenclature for tourmaline was being developed, science had yet to impose its discipline on the study of this group of gems and minerals. This led to the creation of many overlapping and/or misleading names for the individual members in this family of closely-related, naturally-occurring chemicals (minerals) we have come to call "tourmaline." In order to accurately discuss this complex collection of names, we need to divide the world of tourmaline nomenclature into two spheres: one belonging to the world of minerals and mineralogy (as distinguished by their chemistry), and the other belonging to the world of gems and gemology (as distinguished by their color).



Elbaite Tourmaline & Lepidolite from Minas Gerias

Elbaite & Lepidolite - Minas Gerias, Brazil - Zoom

  

Bi-Color Tourmaline - Pala, California - Zoom


In the world of mineralogy there are fourteen different minerals, or species within the family of tourmaline. These fourteen species are all boro-silicates that share a common crystal structure, and form solid solutions with each other. The term "solid solution" means that species can form an infinite number of chemical mixtures, like water and alcohol, only in solid form. This intermixing, or "substitution" of elements has relatively few chemical restrictions. The result of such large variations in chemical composition requires that the mineral name for a species in the solid-solution be used for a range of compositions, and not just the idealized "end-members" that are never actually found in nature.

As recently as the 1960s there were very few species of tourmaline that had been accepted in the world of mineralogy. It was during this period that "lithia tourmaline" began to be referred to as "elbaite," which was followed by a historic trend of reducing the names of various tourmaline-group minerals to a single word, ending with the suffix "ite." The name "elbaite" was derived from the island of Elba because it was the first location where lithia tourmaline was found in situ, as it was formed in the earth, and although the name-change was slow to be accepted, it is now commonly used.



Dichroic Elbaite form Mozambique

Elbaite form Mozambique (photo: Jeffrey R. Smith)

   Dravite Tourmaline form Mozambique

Dravite form Mozambique (photo: Jeffrey R. Smith)


The chemical testing required to distinguish different species of tourmaline has always been difficult, and costly. This may have contributed to the trend of evaluating/naming "species" of tourmaline by color or the geographic origin, rather than by chemistry. In the gemological world, the mineralogical species of tourmaline of the particular interest are dravite, elbaite, liddicoatite and uvite, as well as a theoretical tourmaline-group mineral known as tsilaisite.






Tourmaline Group: Species


Dravite

Dravite is a name for a complex sodium aluminum borosilicate that is a theoretical end-member of tourmaline which has no intrinsic color. Many naturally-occurring dravite tourmalines are brown, brownish-orange or brownish-yellow, which is why brown tourmalines can be mistakenly referred to as "dravite." Due to the rarity of gem-quality dravite, tourmaline that is described as "dravite" may or may not be cut from the actual mineral species dravite, but may instead be a brownish specimen of elbaite.



Orange Tourmaline

Tourmaline from Cabo Delgado (photo: Jeffrey R. Smith)

   Tourmaline Colors from Tanzania

Dravite & elbaite varieties (Photo: © Swala Gems)


The notable exception to dravite's typical brown color is a striking green variety that is called "chrome tourmaline," "chromdravite," or "vanadium dravite," which can have the chromophores chromium (Cr), vanadium (V), or a combination of both chromium and vanadium. There is also an ultra-rare ruby-red variety of dravite that is colored by Fe+3.


Chromdravite (Chrome Tourmaline)

The name "chrome tourmaline" is somewhat of a misnomer, as many chrome dravite tourmalines can also have a predominance of vanadium, and dravite colored primarily by vanadium can have the same green color as dravite colored by chromium. Although the presence of chromium in "chrome tourmaline" does make it appear red when observed through a Chelsea filter (also used to evaluate emerald), this test cannot distinguish between vanadium dravite and green elbaite which is colored by iron (Fe). Therefore, a Chelsea filter is only useful in determining the presence of chromium, and not in distinguishing between mineral species.



Chrome Tourmaline

Chrome Tourmaline (Photo: © Swala Gem Traders)

   Reddish-Brown Dravite Tourmaline

Dravite (photo: Jeffrey R. Smith)


When evaluating a specimen of green dravite it should probably be judged by color, saturation and clarity etc., and not with the use of a Chelsea filter. Of course using only your eye to determine which green is dravite, and which is elbaite can be problematic when you consider how much more expensive chrome dravite tourmaline is when compared with green elbaite that is colored by iron.


Elbaite

Elbaite is a name for a naturally occurring chemical (mineral) that is also grouped with similar chemicals into a category called "tourmaline." It is a complex lithium-bearing sodium aluminum boro-silicate, and is the principle member of the "tourmaline group" that is cut as gemstones. Elbaite tourmaline is typically green or blue-green, and can be colored with iron or copper (cuprian elbaite). For information go to: Paraíba & cuprian elbaite.


Liddicoatite

Liddicoatite is a complex lithium bearing calcium aluminum borosilicate that has many of the same properties and colors of elbaite. These similarities are further enhanced by the interchange of calcium and sodium in tourmaline, and the formation of a solid solution series of compositions between elbaite and liddicoatite. This tourmaline must have at least 50 percent calcium (Ca) ions in it structure to be called liddicoatite.

It is impossible to separated liddicoatite from elbaite by eye, or even by most regular gemological tests, therefore the amount of liddicoate that is actually cut as gems is unknown, but it is probably a relatively small percentage. Multicolored tourmaline with irregular areas of color is sometimes called liddicoatite by the gem trade, but this may be without suitable scientific validation.


Uvite

Uvite is mineral species of tourmaline that is an "end-member" in a solid solution with dravite. Since both uvite and dravite form under similar geological conditions, domains of both species exist in most samples of these tourmalines. Only advanced testing techniques could accurately determine which of the two species dominated, and should therefore be used to describe a given specimen. In the absence of expensive testing, the common usage of "dravite/uvite" may be used.


Tsilaisite

The name "tsilaisite" (from the Malagasy word "tsilai") was proposed for an idealized manganese-rich end-member in the family of tourmaline, although it has never actually been found. One of the deposits for a yellow tourmaline which came the closest to this theoretical end-member was located on the island of Madagascar.

The gem trade quickly picked up on this proposed name, using it to describe a yellow specimen without a brown cast, something which the name "dravite" would also describe. Tsilaisite is not a name that is accepted by mineralogy, but certainly could be if tourmaline of the correct chemical composition is found.


Schorl

For information on schorl (black tourmaline) go to: Schorl






Tourmaline Group: Colors


What's in a Name - Tourmaline Color Classification

Minerals within the tourmaline-group are broken into a dizzying array of names, some being based solely on color, some dovetailing with their correct mineralogical species, and some being based on marketing. Names such as achroite, Paraíba, rubellite, tsilaisite and verdelite are all used to describe different varieties/colors of elbaite or dravite. Those within the field of mineralogy may take exception to their use because the chemical differences between them, or lack thereof, does not qualify them as separate minerals, using the mineral suffix "ite." However, some of these names are well established in the field of gemology, having both economic, and historic significance.



Schorl Tourmaline

Schorl Tourmaline (USGS)

   Pink Elbaite

Pink Elbaite (Public Domain)


In addition to the inherent problems associated with this kind of variant, and somewhat arbitrary naming criteria, there are also historic names that associate a particular color of tourmaline with completely different minerals/gemstones. One such example is "Brazilian emerald," which is used to describe a rich green variety of tourmaline. This type of cross-identification, and therefore, misidentification, is generally frond on by the industry as being deceptive. Within the gem-trade, each color of tourmaline is associated (correctly or incorrectly) with its own name:


Common Gem Name Mineral Species Primary Colors (*primary chromophore)
Anchorite Elbaite colorless
Bi-Color Tourmaline Liddicoatite green or gray to red transition
Cat's-Eye Tourmaline Elbaite green, pink, chatoyant
Chrome Dravite Dravite green (*chromium, vanadium)
Chrome Tourmaline Dravite green (*chromium, vanadium)
Cuprian Elbaite Elbaite cyan, green, pink, purple, red, yellow (*copper)
Dravite (generic name) Dravite, Elbaite brown, orange, yellowish-brown
Elbaite Elbaite blue, brown, green, multicolored, purple, red
Indicolite (indigolite) Elbaite pure blue, dark indigo blue (*iron)
Liddicoatite Liddicoatite, Elbaite blue, brown, green, multicolored, purple, red
Neon Dravite, Elbaite, etc. any bright, vibrantly colored tourmaline
Paraíba Elbaite (Cuprian) neon blue, Windex blue, green-blue (*copper)
Paraíba-like (African Paraíba) Elbaite (Cuprian) neon blue, Windex blue, green-blue (*copper)
Pink Tourmaline Elbaite pink
Rubellite (ruby tourmaline) Elbaite magenta, red (*manganese)
Schorl (generic name) Schorl charcoal gray, black
Tsilaisite Elbaite brown, yellow (*manganese)
Uvite Uvite green or orange-brown
Verdellite Elbaite green, chatoyant (*iron)
Watermelon Tourmaline Liddicoatite green or gray to red transition
Tri-Color Tourmaline Liddicoatite green or gray to red transition

Tourmaline Color Chart © AllAboutGemstones.com 2009



Once a name has taken hold in the marketplace it is nearly impossible to correct, as the name takes on economic and mythical significance. Unfortunately, all of this has led to a considerable amount of confusion, both in the gemological world, and by extension, to the consumer.


Bi-Colored & Watermelon Tourmaline - The Rainbow Gem

Multicolored, bicolor, and tricolor tourmaline with irregular zones, or bands of color are generically identified by the gem trade as litticoatite (see "Litticoatite" section above), but they can also be elbaite. For over 100 years some of the finest examples of banded multicolor liddicoatite have been found in the location of its discovery (its "type location"), in Anjanabonoina, Madagascar, and this is why the association has stuck, but elbaite can have the same colors, and distribution of color.

Multicolored tourmaline owes its "rainbow" coloration to trace impurities such as chromium (red), iron (dark blue, dark brown), lithium (green, pink), manganese (pink), schorl (black or gray) and vanadium (green). So-called "watermelon tourmaline" (below, right) is known for its concentric bands of color featuring a reddish, pink, purple, or magenta center surrounded by a whitish zone enclosed in a forest-green "rind" area.



Bi-Color & Watermelon Tourmaline

Bi-Color & Watermelon Tourmaline

   Tri-Color Tourmaline

60 ct. Tri-Color Tourmaline (photo: Jeffrey R. Smith)


Pink and lilac hues in multi-colored tourmaline could also be due to the presence of lepidolite which is a phyllosilicate mineral of the mica group, and a secondary source of lithium. Certain varieties of tourmaline will display dichroism, reflecting and splitting visible light into different wavelengths in a prism effect. Tourmaline is double refractive, and typically exhibits moderate to strong dichroism or pleochroism.

An ancient Egyptian legend explained the array of colors found in multi-colored tourmaline varieties thusly: "On it's long journey from the middle of the earth up toward the sun, the tourmaline traveled along a rainbow collecting all of its colors," and it is this multi-colored (aka "parti-colored") quality that endowed tourmaline with the nickname of the "rainbow gemstone."



Pink Tourmaline

So-called "pink tourmaline" is actually a pink variety of elbaite that has lithium (Li) as its main chromophore. A principle source of pink tourmaline can be found in Afghanistan. Afghan tourmaline can be found among metamorphic pegmatite veins of gneiss, mica schist, and quartzite located in in the Panshir Valley, north-east of the capital city of Kabul.


Blue Tourmaline (Indicolite)

Pure blue tourmaline is one of the rarest color varieties in the tourmaline mineral group, which has been identified by the commonly-used names "indicolite," or "indigolite." The species of tourmaline that is most often associated with indicolite is elbaite, but the calcium analog of elbaite, liddicoatite, can also be blue.

Iron is a common chromophore in blue elbaite, but to create the blue color in elbaite it must form an intervalence charge transfer reaction (IVCT) between its two oxidation states (Fe+2 and Fe+3), and that is not common. Blue tourmaline, besides being rare, also tends to be dark and have an overly saturated principle axis (c) that can be completely closed to light transmission. This is an effect of being colored by an IVCT. Heating is sometimes used to lighten Indicolite and make it more transparent.

Blue tourmaline can have a slight green cast, which is also caused by iron, and should prevent the gemstone from being called "indicolite." Opinions in the gem trade vary on the fine line between "indicolite" and the considerably more common blue-green tourmaline, but blue-green can be one of tourmaline's most beautiful colors so it is still highly valued.

Blue tourmaline with a purplish cast is even rarer than pure blue tourmaline, and is mostly limited to cuprian elbaite (colored by copper), producing neon blues that are highly prized, and typically more expensive the indicolite.

Indicolite can mimic the color of blue sapphire, blue topaz, and other similarly-colored gemstones. Indicolite occurs in small quantities in a variety of locations throughout the world, and special notice should be given to Brazil which produces most of the indicolite. Namibia also had produced small amounts of high quality Indicolite over the years.


Paraíba (Cuprian Elbaite) Tourmaline

For information go to: Paraíba


Ruby (Red) Tourmaline

For information go to: Rubellite






Tourmaline Group: Properties & Occurrence


Tourmaline Group Crystallography

Tourmaline is a complex silicate mineral (acentric rhombohedral borosilicate) with a parallel and striated crystal habit, growing elongated crystals with each prism face having pronounced vertical striations and subhedral formations. The elongated crystals have asymmetrical terminations known as hemimorphism, and tourmaline crystals are rarely euhedral.


Tourmaline Crystal Structure

The X-site is a nine-coordinated trigonal antiprism, located along the three-fold axis of symmetry. It is most commonly occupied by Na and Ca (with minor K), or can be vacant [5]. According to the work of Frank C. Hawthorne of the University of Manitoba (Department of Geological Sciences) in Winnipeg, and Darrell J. Henry of Louisiana State University, the chemical diversity and structural requirements of tourmaline give rise to the possibility of over 27 additional end-member species that could exist in nature.


Tourmaline Group Chemistry & Physical Properties

Crystal System trigonal
Crystal Habit parallel elongated, striated - asymmetrically terminated
Specific gravity (SG) 3.03 - 3.25
Mohs Hardness Scale 7.0 to 7.5
Toughness fair to good
Fracture uneven, small conchoidal
Cleavage indistinct
Streak white
Chemical Composition XY3Z6(T6O18)(BO3)3V3W (see: full mineral list below)

Tourmaline Group Optical Properties

Optical Properties double refractive, uniaxial negative
Refractive Index 1.624
Birefringence -0.018 to -0.040
Dispersion .017
Pleochroism moderate to strong dichroism
Surface Luster vitreous to resinous
Diaphaneity transparent to translucent
Gem Color all colors, black or gray, colorless, bi-colored, tri-colored

The tourmaline group consists of fourteen individual minerals that can be separated into three main groups based on the dominant occupancy of the X site. These groups are the alkali-tourmaline group (Na), the calcic-tourmaline group (Ca), and the X-site-vacant tourmaline group.


Chemical Composition of All 14 Minerals in Tourmaline Group

Alkali Tourmaline
Buergerite NaFe3+3Al6Si6O18(BO3)3O3F
Chromdravite NaMg3Cr6Si6O18(BO3)3(OH)4
Dravite NaMg3[Cr,Fe3+]6(BO3)3Si6O18(OH)4
Elbaite Na(Mg,Fe,Mn,Li,Al)3Al6(BO3)Si6O18(OH,F)4
Olenite NaAl3Al6(BO3)3Si6O18(O)3(OH)
Povondraite NaFe3+3Fe3+6(BO3)3Si6O18(O)3(OH)
Schorl NaFe2+3Al6(BO3)3Si6O18(OH)4
Vanadiumdravite NaMg3V6Si6O18(BO3)3(OH)4
Calcic Tourmaline
Hydroxy-Feruvite CaFe2+3[Al5Mg](BO3)3Si6O18(OH)4
Liddicoatite Ca(Li2Al)Al6Si6O18(BO3)3(OH)3F
Uvite CaMg3[Al5Mg](BO3)3Si6O18(OH)4
X-Site Vacant Tourmaline
Foitite (Fe2+2Al)Al6Si6O18(BO3)3(OH)4
Magnesiofoitite (Mg2Al)Al6Si6O18(BO3)3(OH)4
Rossmanite (LiAl2)Al6(BO3)3Si6O18(OH)4

Tourmaline Occurrence

Some of the finest specimens of tourmaline are found in association with igneous rock pegmatite dikes and within alluvial deposits or cascalho, located in the area around Virgem da Lapa in Minas Gerais, Brazil. Ancient rivers have washed through gem deposits scattering stones throughout this region. Some of the mines that have produced the greatest specimens of gem crystal tourmaline and aquamarine are the primary deposits of Araçuaí, Corrego do Urucum, Cruzeiro, Golconda, Jonas, Limoeiro, Medina, Pedra Azul and Xanda.

Other significant sources for tourmaline are found within primary hard-rock deposits in Afghanistan and Pala, California, as well as secondary alluvial deposits in Sri Lanka (Ceylon), Nigeria and Namibia.


Technical assistance provided by Bruce A. Fry





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Bibliography and Reference on the Tourmaline Group of Minerals


1. American Mineralogist, Color in Cuprian Elbaite www.minsocam.org

2. F. Hawthorne, D. Henry, Tourmaline Classification Scheme . www.geol.lsu.edu

3. American Mineralogist, The Origins of Color in Minerals www.minsocam.org

4. The Mineralogical Record, A Mineral Collector's Knowledge Database www.minrec.org

5. Darrell Henry, Tourmaline Structure . www.geol.lsu.edu

6. Cally Hall, Smithsonian Gemstones . Simon & Schuster.

7. Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition

8. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition

9. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press




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