Lets Talk Gemstones

By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962


The Tourmaline Group

[A CYCLOSILICATE]

BUERGERITE
CHROMDRAVITE
DRAVITE
ELBAITE
FERRIDRAVITE
LIDDICOATITE
TSILAISITE
SCHORL
UVITE

Our trip to Tucson this year provided the opportunity to see a number of the species of gemstones in quantities that permitted comparison not usually available at other sites. The range of quality, color, and price was extensive. Knowledge of the properties of a gemstone and care in selection are imperative in order to choose an appropriate gem. The popular tourmaline group presents wide variations in color and quality from which to select outstanding gems at very reasonable prices.

Tourmalines are classified as silicates. Silicates are minerals composed of SiO4 tetrahedra arranged in various structural configurations that incorporate other chemical elements in the structure. In tourmaline, the tetrahedra form rings, where several different elements in various proportions can be accommodated. Such a ring structure is known as a cyclosilicate. The tourmalines are a group of extremely complex aluminum borosilicates (boron is the one constant element in combination with the SiO4 tetrahedra) that crystallize in the hexagonal (trigonal) crystal system. Numerous differences in physical and optical properties exist in the various species. Elbaite and liddicoatite form a continuous solution series, as do uvite and dravite. The nine species most likely to be encountered by those involved in the jewelry industry are the subject of this article. Elbaite and liddicoatite are the species most used as gem material.

Some of the transparent color varieties of tourmaline are identified by commonly used descriptive names. Achroite, meaning “without color,” denotes the colorless variety. Pink and red stones are often called rubellite. An exceptionally intense electric pink material is found in California. The strong pleochroism of the unique deep greenish-blue Indicolite (indigolite) may cause the gem to appear green or seem to lose transparency. A deposit near Newry, Maine is the source of some of the finest blue-green and red tourmalines. All shades of green tourmalines are sometimes called verdelite. Traces of chrome impart an especially vivid color to the chrome-green tourmaline. The violet-red colors (prevalent from a deposit in Siberia) are known as siberites. Bi-color and parti-color crystals show a variation of color along the length of the crystal.

Crystals found on the island of Elba in the Mediterranean Sea often exhibit striking bands of color. Simon and Schuster’s Guide to Gems and Precious Stones refers to “moor’s head” crystals from this source. In A Guide in Color to Precious and Semiprecious Stones, Jaroslav Bauer and Vladimir Bouska state “One end of the crystal may be green, then the colour may change into a yellow or even a colourless zone, and the other end may be black.” The Illustrated Encyclopedia of Minerals and Rocks by Dr. Jiri Kourimsky describes them as “a common occurrence” having “black ends, but a green and pink core.” A similar crystal exhibiting a red termination is called a “turkhead.” On page 180 of the Simon and Schuster volume, a photograph of a four-carat Brazilian cabochon gem shows an extremely sharp division of its reddish- brown and greenish-blue colors. Watermelon tourmaline, usually from Brazil, is an apt name when the variation of colors occurs in concentric bands with a reddish-pink center surrounded by a white zone and enclosed in a green “rind.” South Africa produces material where the color sequence is reversed. Concentrations of fibrous or acicular inclusions, commonly in green and pink materials, cause the “cat’s-eye” effect in some stones cut en cabochon. Even the change of color phenomenon is found in tourmaline. The alexandrite-like gems change from yellowish-green in balanced white light to reddish-orange in incandescent light.

Tourmaline possesses strong pyroelectric and piezoelectric properties. [The following is quoted from the American Geological Institute’s Glossary of Geology: “piezoelectric effect; In certain crystals, the development of an electric potential in certain crystallographic directions when mechanical strain is applied”, also pyroelectricity; The simultaneous development, in any crystal lacking a center of symmetry, of opposite electric charges at opposite ends of a crystal axis, due to certain changes in temperature.] The piezoelectric property makes tourmaline useful in the manufacture of gauges to measure transient blast pressures. Its pyroelectric nature was discovered as early as the seventeenth century, when long prisms of “Brazilian emeralds” were brought to Europe by Dutch traders. George Kuntz, in The Magic of Jewels and Charms, tells of children using such “aschentreckers” warmed by the sun to attract and repel straw and ashes.

The electrical properties of the stone also intrigued Benjamin Franklin. When “some ingenious gentlemen from abroad” denied its negative polarity, he concluded the examined specimens were improperly cut. He wrote the following in a letter, dated 7 June 1795, thanking Dr. William Haberden for two stones, “the positive and negative planes having perhaps been obliquely placed. To obviate this, I suggest that the positive and negative sides should be accurately determined before the operation of cutting begins.”
Today, tourmaline’s distinct pleochroism is a more important factor to the facetor. The weak dichroism of pale colored crystals allows the cutter a wide latitude on the orientation of facets. With dark material, the placement of the table facet parallel to the optic axis of the crystal is often necessary to obtain a lighter more desirable color. Some deep-colored gems, especially the dark reds, blues, and greens, exhibit a unique loss of transparency if the table facet is oriented perpendicular to the prism axis. Deep green and brown crystals exhibit strong pleochroism. In instances when the strength of the absorption of the o-ray is sufficient to plane-polarize light, only one edge of absorption may be visible on the refractometer. Such tourmalines may appear isotropic.
Simon and Schuster’s Guide to Gems and Precious Stones tells of an unusual property that is characteristic of mid-green tourmaline gems. A unique optical effect is exhibited when such stones are given a rectangular cut. The way in which light is reflected from the pavilion facets causes distinctive alternate longitudinal lines of lighter and darker color.       

Because tourmaline occurs in such a wide range of colors, it may be confused with many other transparent gemstones. Its strong pleochroism and similar refractive indices to andalusite dictate careful examination of greenish yellow-brown stones to avoid misidentification. Tourmaline can also be confused with amethyst, citrine, apatite, peridot, topaz, danburite, idocrase, synthetic spinel, glass, and other materials. Tourmaline’s lack of cleavage is a factor in distinguishing it from hornblende. Simon and Schuster’s Guide to Rocks and Minerals makes note of the fact that synthetic tourmaline, which is now available, is identifiable only by laboratory tests. However, in their volume Guide to Gems and Precious Stones, it is stated that rubellite, indicolite, and green tourmaline “are neither imitated nor produced synthetically.” The Singhalese word “turamali”, meaning mixed-colored stones, is given by Arem in the Color Encyclopedia of Gemstones as the source for the name tourmaline. It is interesting to note that Bauer and Bouska spell the word “toramalli” and define it as “carnelian”, an alternate spelling of carnelian. Carnelian is translucent reddish chalcedony.

The pegmatites of Minas Gerais, Brazil have replaced Sri Lanka’s alluvial deposits as the source of most of the world’s tourmaline material. Especially vibrant “cotton candy” colored gems come from deposits in and near Paraiba. These “paraibas” command premium prices. Tourmaline deposits in Nigeria and Namibia yield exceptionally fine crystals that may rival the intense color of the paraibas. Madagascar’s crystals of various colors often resemble those from Brazil. Fine red “Siberian rubies” are among the many hues found in the coarse granites of the Urals near Murzinka and Sverdlovsk. The largest gem quality crystals are recovered in Mozambique. A magnificent 42-cm long, rich red specimen from Mozambique’s Muiiane area is on display in the museum in Lourenco Marques.

With the exception of schorl, of pegmatitic origin, tourmaline is a late pegmatitic hydrothermal product. Inclusions of protogenetic minerals, such as apatite, pyrite, colorless quartz, mica crystals and hornblende, are common. Concentrations of mineral fibers, perhaps amphiboles, cause the cat’s-eye phenomenon in some tourmalines. Gas may fill small internal fractures in red tourmaline, and very flat films can reflect light.

Two specific types of inclusions, however, developed syngenetically in the hydrothermal environment. Growth tubes, resembling gramophone needles parallel to the crystal axis, are long liquid filled channels extending from tiny crystals. This “mother liquor” may be accompanied by a secondary liquid as well as a gas bubble. These tubes occur mainly in blue, green, red, and pink material, and if densely packed, can cause the cat’s-eye effect. Diagnostic “trichites,” which are also 2-phase inclusions, differ sharply from growth tubes and are common to all tourmalines. The irregular hair-fine networks formed by partially healed crevice surfaces consist of tiny capillaries and vesicles filled with syngenetic secondary fluid. Tourmalines develop in an especially complex chemically rich environment. So few are completely free of inclusions. Black acicular tourmaline is sometimes found as protogenetic inclusions in transparent “tourmalated quartz.”

The lovely colors of rubellite, the green “chrome tourmalines”, the vibrant “paraibas”, and the exquisite gems from Nigeria and Namibia are most in demand today. The consumer has an extensive palette of hues from which to choose. With hardness comparable to quartz and the lack of a perceptible cleavage, tourmaline is an excellent choice for all types of jewelry, including rings and bracelets.

In the following, each species is listed separately, and information specific to each one is presented. Properties common to all species are shown also shown.




Buergerite    
NaFe3Al6B3Si6O30F        
Sodium aluminum borosilicate

A bronze iridescent schiller lying just below the crystal surface is characteristic of this yellow-brown to black specie of tourmaline. The specific gravity can vary from 3.29 to 3.32. The ordinary ray of the refractive index is 1.735, while the extraordinary ray reading can fall between 1.655 and 1.670. This results in a birefringence range of 0.065 to 0.080. The pleochroic colors of yellow-brown and pale yellow are the norm. Buergerite is found in rhyolite near San Luis Potosi in Mexico. Its name honors the crystallographer and research scientist, Professor Martin J. Buerger.

Dravite
NaMg3Al6B3Si6O27(OH)3(OH,F)
Magnesium aluminum borosilicate

This yellow-brown to black tourmaline specie forms a series with schorl and with elbaite as well as with uvite [Simon and Schuster’s Guide to Rocks and Minerals]. Dravite crystals are usually found in contact metamorphic and metasomatic rocks, pegmatites, and crystalline limestones. Deposits in the Carinthian district of Drave, Austria are the source of its name. It occurs in Crevoladossola, Novara, Italy and at Gouverneur, in St. Lawrence County, DeKalb and Pierrepont, New York, [USA]. Kenya produces both red and yellow material. Deposits yielding excellent intense red crystals exhibiting the properties of refractive indices [e = 1.623, o=1.654, density = 3.07] and [e = 1.626, o = 1.657] and a specific gravity of 3.04 are located there. The yellow crystals present slightly lower readings [e=1.619, o = 1.642 with S.G. = 3.04]. Red crystals from near Chipata, Zambia exhibit properties quite similar to the Kenyan red material. Crystals with pyramidal form are found in Australia near Yinnietharra. Numerous pairs of pleochroic colors can be displayed in dravite: colorless/yellow, light yellow/orange-yellow, yellowish to pale brown/medium to deep brown, deep green/yellow-green, and blue-green/yellow-green.


Chromdravite
NaMg3Cr6(BO3)3Si6O18(OH)4        
Sodium magnesium borosilicate

Aluminum is absent, and chrome is present, which give an intense green color to chromdravite found in the central Karelian region, located near the eastern border of Finland. The refractive indices are among the highest for the tourmalines [o = 1.778   e = 1.772]. The pleochroic colors are dark green and yellow-green, respectively. The density of 3.39 – 341 is the greatest of the several tourmaline species. The chemical composition accounts for its name.

Ferridravit
(Na,K)(Mg,Fe+2)3Fe6+3(BO3)3Si6O18(O,OH)4
Iron Magnesium borosilicate

Aluminum is also absent from this tourmaline specie. Its colors range from brown to dark yellow-green. Pleochroic colors of deep-brown to deep olive-green/pale brown to pale olive-green are the norm. Optical properties include a birefringence of 0.057 with refractive index readings of e = 1.743 and o = 1.80 to 1.82. With a variation of 3.18 to 3.33, the density is a bit less than that of chromdravite.

Elbaite
Na(Li,Al)3Al6B3Si6O27(OH)3(OH,F)    
Sodium lithium aluminum borosilicate

Elbaite is one of the two species of tourmaline most used as gem material. Its name is derived from the Isle of Elba, Italy, the source of some of the finest of these lithium-rich tourmalines. The extensive deposit at Newry, Maine [USA] yields stunning colors of pink, red, blue, blue-green and green crystals. An uncommon pastel pink is found at Pala, California. Nuristan, Afghanistan is known for superb emerald green, blue, and pink material. Minas Gerais in Brazil produces gem quality crystals in large sizes and a wide range of colors, as well as watermelon, bi-color, and tourmaline cats-eye material. The Jonas Lima mine is the source of fine, extremely large cranberry-colored crystals. Excellent dark red crystals are found at Ouro Fina.
The rubellite from Madagascar is prized, and the Somabula Forest region in Zimbabwe produces excellent elbaite. Various pale colors and bi-colors are produced at Alta Lingonha, Mozambique. Violet, blue and red crystals are extracted from the decomposed granites at Nerchinsk and Mursinka in the Urals. Pink elbaite is found with red crystals at Mogok in Myanmar [Burma]. The deposit near Klein Spitzkopje, Otavi, Namibia yields various colors including numerous shades of green. The green elbaite crystals produced in Kashmir, India exhibit a specific gravity of 3.05, refractive indices of 1.622-1.643 and birefringence of 0.021. Deposits at Usakos, Namibia yield excellent green chrome tourmaline. The vivid green crystals from Tanzania contain traces of chrome and vanadium. Zambia is the source of a yellow manganese-bearing elbaite resembling tsilaisite in color and chemical composition. Color-zoned material is found at Haddam, Connecticut [USA] and at Glenbuchat, Aberdeenshire, Scotland.

The density range of elbaite is 2.84-3.10 with the norm of 3.05. Refractive indices [o =1.619-1.655   e =1.603-1.634] result in a birefringence that can vary from 0.013-0.024. Pleochroic colors vary according to the body color of the material.    

Gem Color    o     e
Pink- red    pink    colorless or pale pink
Blue    medium blue    colorless, pink or violet
Blue-green    bluish green    pale green to violet
Green    green    yellowish green to yellow



Liddicoatite
Ca(Li,Al)3Al6B3Si6O27(OH)3(OH,F)       
Calcium lithium aluminum borosilicate

It was only in recent history that this calcium-rich lithium-bearing tourmaline specie was differentiated from elbaite and named to honor the late Richard T. Liddicoat. Madagascar is the source of the sometimes extremely large crystals. Material exhibiting multi-bands of several colors and fine rubellite are extracted from these deposits. Like elbaite, the color-range of the normally medium to lighter hued liddicoatite is extensive. Its physical and optical properties exhibit few variations from the norm. 3.02 is the usual density reading. The ordinary ray of its refractive index varies little from 1.637, while the extraordinary ray is constant at 1.621. Birefringence is 0.016. The pleochroic colors mimic those of elbaite. However, brown crystals occur and exhibit o = brown and e = pale brown hues.


Schorl
NaFe32+Al6(BO3)3Si6O18(OH)4    
Sodium iron aluminum borosilicate

The old mining term denoting any of several dark brown, green, blue-black or black rocks and minerals has become the accepted name for this common tourmaline specie that forms a series with dravite. Sites in England are a major source for the material, but distribution in granite pegmatites is worldwide. Its frequently hemimorphic, heavily striated prismatic crystals can attain several feet in length. Acicular crystals housed in transparent quartz gives rise to the term “tourmalated” quartz. Schorl’s hardness – 7.25 – is mid-range for tourmaline with a specific gravity of 3.10 – 3.25. Its refractive indices are a bit above normal with readings of 1.62 – 1.69. During the Victorian era, faceted schorl, as well as jet, was used extensively in mourning jewelry. Today, the collectors of unusual gemstones are its principal market.


Tsilaisite
Na(Mn,Al)3Al6(BO3)3Si6O18(O,OH,F)4
Sodium manganese aluminum borosilicate

Manganese oxide can make up as much as 9.2 percent of the chemical composition of this very rare bright yellow to red tourmaline material found in Zambia. Its birefringence range of 0.023-0.028 is the function of refractive index readings from 1.622 to 1.648. It exhibits pleochroic colors of yellow-brown for the ordinary ray and a vivid yellow for the extraordinary ray. The average specific gravity is 3.13. Confusion with elbaite is a distinct possibility unless the source and its chemical composition can be established.

Uvite
CaMg3(Al5Mg)B3Si6O27(OH)3(OH,F)
Calcium magnesium aluminum borosilicate

The extensive mineral deposits at Franklin and Hamburg, New Jersey and near DeKalb and Gouverneur, New York are sources of the dark brown uvite crystals. A number of years elapsed before uvite was identified as the calcium rich end member of the solid solution series formed with dravite. Its density range of 3.01-3.09 is a bit lower than that of dravite. The same is true of the optical properties range. Refractive indices of o =1.632-1.660 and e =1.612-1.639 produce a birefringence range of 0.017 – 0.021. The pleochroic colors are quite similar to those exhibited by dravite. Collectors of unusual gems may acquire faceted stones, but uvite is practically unknown in the jewelry trade.

COMPOSITION:    See individual specie
CLASS:    Silicates
GROUP:    Tourmaline
SPECIES:    Buergerite; Chromdravite; Dravite; Elbaite; Ferridravite; Liddicoatite;     Schorl;     Tsilaisite;   Uvite
VARIETY:    By color and by the following designations:
        Achroite – colorless
        Dravite – yellowish-brown to dark brown
        Indicolite (indigolite) – all blue tones
        Rubellite – pink to red – possible violet tint
        Siberite – lilac to violet-blue
        Schorl - black
CRYSTAL SYSTEM:    Hexagonal [Trigonal]
HABIT:    Elongated Prismatic- usually exhibits different termination forms at opposite ends of vertical axis when doubly terminated - vertical striations; Columnar; Acicular; Massive
CLEAVAGE:    Imperceptible
STREAK:    White
FRACTURE:    Conchoidal; uneven, Brittle
FRACTURE LUSTRE:    Frequently resinous     
LUSTRE:    Vitreous to resinous
DIAPHANEITY:    Transparent Translucent       Opaque
COLORS:    All
PHENOMENA:    Chatoyancy, Change-of color
SPECIFIC GRAVITY:    See specific specie
HARDNESS:    7.0 – 7.5
TOUGHNESS:    Good    Brittle
REFRACTIVE INDICES:    Varies by specie     o = 1.619 to 1.82    e = 1.603 to 1.772
BIREFRINGENCE:    See specific specie
OPTIC CHARACTER:    Uniaxial   (-)   Arem
DISPERSION:    0.017
PLEOCHROISM:    See specific specie
LUMINESCENCE:    Weak- variable – Blue, Newry, Maine– SW-chalky blue/deep- blue Pink, Brazil – SW-pale blue or light violet Yellow; green; brown –Tanzania – SW – Strong yellow
SPECTRUM:    Usually faint – Not diagnostic   
CHELSEA FILTER:    No information
AQUA FILTER:    No information
SOLUBILITY:    Insoluble in acids
THERMAL TRAITS:    AVOID THERMAL SHOCK       
    Fusibility – Variable with composition
    Lithium bearing varieties - infusible
    Iron-rich varieties – fusible with difficulty
    Magnesium-rich varieties – fusible at 3
    Brief green flame when fused with boron flux       
TREATMENTS:    Heat to 450oC to achieve paler hues.     Difficult to detect. Causes brittleness. Opticon for fracture filling. Irradiation uncommon.
INCLUSIONS:    See information above