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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.
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: SpeciesDraviteDravite 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.
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.
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. ElbaiteElbaite 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. LiddicoatiteLiddicoatite 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. UviteUvite 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. TsilaisiteThe 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. SchorlFor information on schorl (black tourmaline) go to: Schorl Tourmaline Group: ColorsWhat's in a Name - Tourmaline Color ClassificationMinerals 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.
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:
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 GemMulticolored, 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.
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 TourmalineSo-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) TourmalineFor information go to: Paraíba Ruby (Red) TourmalineFor information go to: Rubellite Tourmaline Group: Properties & OccurrenceTourmaline Group CrystallographyTourmaline 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.  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
Tourmaline Group Optical Properties
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
Tourmaline OccurrenceSome 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
Bibliography and Reference on the Tourmaline Group of Minerals 1. American Mineralogist, Color in Cuprian Elbaite 2. F. Hawthorne, D. Henry, Tourmaline Classification Scheme 3. American Mineralogist, The Origins of Color in Minerals 4. The Mineralogical Record, A Mineral Collector's Knowledge Database 5. Darrell Henry, Tourmaline Structure 6. Cally Hall, Smithsonian Gemstones 7. Walter Schumann, Gemstones of the World 8. Renee Newman, Gemstone Buying Guide 9. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy | Copyright © 2012 AllAboutGemstones.com. All rights reserved. | | | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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: The Tourmaline Group
