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INDEX
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An Introduction To The Tourmaline Group
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The eleven members of the tourmaline group show a huge variety of colors, including red, orange, yellow, green, blue, violet, pink, colorless, white, grey, black, and brown. In fact, tourmaline shows a greater range of colors than any other gem. Crystals
are often color-zoned, with either different colored cores and exteriors, like elbaite 'watermelon' tourmaline with a red interior and green exterior; or with bands of color perpendicular to the length of the crystal, like pink elbaite crystals with
blue caps. When transparent and vibrantly colored, elbaite is a popular gemstone, with global annual sales of millions of dollars. Stones of several hundred carats have been cut.
This is an excellent group for a collector to specialize in, though large or particularly gemmy crystals are expensive. Individual mines can produce literally tons of tourmaline. Identification of individual species can be difficult. Most specimens sold
as tourmaline are the species elbaite. Dravite, elbaite, schorl, and uvite are readily obtainable, but the other species may be challenging to acquire. Dealers have crystals in all sizes
and colors from many localities. Matrix specimens, or crystals associated with other crystallized minerals are particularly desirable. Also desirable are the unusual 'bent' (broken and rehealed) crystals, and doubly terminated crystals showing
hemimorphism. Crystals showing color zoning are also highly collectible. Classic localities to look for include Elba, Italy, and the famous old pegmatites of Maine.
Tourmaline group members are borosilicates of the general formula:
WX3Y6(BO3)3Si6O18(O,OH,F)4, where
W = Ca, K, Na
X = Al, Fe2+, Li, Mg, Mn2+
Y = Al, Cr3+, Fe3+
Species of this group crystallize in the Hexagonal system, Trigonal Division. They are usually elongated prisms, occasionally stubby, and hemimorphic (with different terminations on each end of the crystal).
Crystals are often triangular in cross-section.
Physical properties: Hardness of 7, brittle, conchoidal fracture, no cleavage, specific gravity a little over 3, transparent to translucent, colorless streak, vitreous to resinous luster. Occasionally pleochroic or
chatoyant. Strongly piezoelectric and pyroelectric. Insoluble in acids.
Geological Occurrence: Common in igneous and metamorphic rocks, and in sedimentary rocks as detrital or authigenic grains. Very common in pegmatites and greisen, occasionally in marbles.
Tourmaline Group Members Briefly Described
Buergerite - Na Fe Al borosilicate - Dark brown to black crystals, in divergent sprays, to 4 cm long. Uncommon, from Mexquitic, San Luis Potosi, Mexico, where it occurs in rhyolite.
Chromdravite - Na Mg Cr Fe borosilicate - A rarely seen tourmaline, green-black, in acicular crystals to 0.1 millimeters. Found in micaceous metasomatic clay-carbonate rocks in the Onezhkii basin, central Karelia,
Russia.
Dravite - Na Mg Al borosilicate - Crystals are stubby to long prismatic, often striated vertically. Colors include brown, black, greenish-black, dark red, and pale bluish-green to emerald green. Found in many places, but the best and most famous crystals
are probably those from Yinnietharra, Western Australia, where sharp complete doubly-terminated brown crystals to the size of a softball have been found. Not hard to obtain.
Elbaite - Na Al Li borosilicate - This is the most famous tourmaline group member, occurring in many colors, including red, green, blue, yellow, and rarely colorless. Crystals are short to long prismatic, vertically striated, occasionally acicular or
other habits. Elbaite most often occurs in granite pegmatites, and is found in fine crystals at numerous localities all over the world. Famous localities include the Himalaya Mine in California; Mount Mica in Maine; Elba Island in Italy; and many mines in
Brazil. Crystals to 1.6 meters long have been found.
Feruvite - Ca Fe Mg Al borosilicate - Rare, dark brown-black grains to 2 millimeters, from Cuvier Island, New Zealand. Formed by hydrothermal replacement of silicates in a pegmatitic rock.
Foitite - Fe Al borosilicate - Bluish-black, prismatic striated crystals with triangular cross-section, found in large crystals in California pegmatites.
Liddicoatite - Ca Li Al borosilicate - Found in stout crystals, up to 25x10 centimeters, striated along their lengths. Astonishing and complex multi-colored color-zoning, in triangular patterns, is exposed when
crystals are cut perpendicular to their elongation. These huge crystals are found in pegmatites in various parts of Madagascar. Not often available.
Olenite - Na Al borosilicate - Rare, vitreous pale pink. Found as the outer zone, to 3 millimeters thick, of acicular crystals with elbaite cores. From pegmatitic veins crosscutting Precambrian metasediments, on the Oleny Ridge, Kola Peninsula,
Russia.
Povondraite - Na Mg Fe borosilicate - Very rare, in fractures and lining cavities in schist metamorphosed from sedimentary rocks, at the San Francisco Mine, near Villa Tunari, Alto Chapare, Cochabamba, Bolivia. Black, nearly opaque crystals, equant and
distorted, in subparallel aggregates, to several millimeters.
Schorl - Na Fe Al borosilicate - From black to brownish to bluish black. Superb lustrous sharp crystals are found in many places, especially California, Brazil, Pakistan and Afghanistan.
Uvite - Ca Mg Al borosilicate - Crystals are often equant, as well short to long prismatic. Colors include red green and brown. Found in New York State, as well as Myanmar(formerly Burma), Sri Lanka, Kenya, and other places. Finest specimens are bright
red or green crystals on quartz or magnesite crystals from Brumhado, Brazil.
Further Reading:
"Mineralogy," John Sinkankas, 1964
"The Tourmaline Group," R.V. Dietrich, 1985
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Copyright, 1997 by Tim Jokela, Jr.
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