Gemmology Canada - Special Issue
by Rona Johnston, A.G. (C.I.G.), Winnipeg
All stages of imaginable colour possibilities are captured by this gemstone which, in addition to white and black, embraces every hue to be found in the spectrum, and not only in pure tones but in all fine nuances of innumerable shades, transition, and mixtures. Should a collector set himself the task of amassing all the colorings of tourmaline, he would find a lifetime insufficient to incorporate the thousands upon thousands of ever differently tinted specimens into his collection.1
Tourmaline, is one the most fascinating of the gemstones species, incorporating colours from black to vivid greens, exhibiting unusual bri/tri colour features. Unlike it's more familiar gem members, corundum and diamond, little myth or romance has attached itself to this stone. Though it was named and known as "tourmaline" as early as the 1700's, it's identification as an actual species was only recently. According to R.V. Dietrich, the name "tourmaline" appears to have been derived from the Sinhalese term turmali, which was applied by the ancient Ceylonese merchants to mixed gemstones of unproved identities.2
Tourmaline was first discovered on the Isle of Elba (hence the name Elbaite which is still used). This nomenclature is presently used to cover the majority of tourmaline colourings including pink, green, yellow, violet, colourless, etc. Liddicoatite is often used to describe Elbaite material. Black/brownish material is known as "Schorl", red as "Rubellite", blue as "Indicolite", green as "Verdelite". Dravite and Uvite (black/brown/dull greenish) are rarely of gem quality and generally less colourful make-up. Buergerite is a bronzy brown. Achroite is the colourless variety. Nowadays, coloured tourmaline is referred to by descriptive adjectives. The bi/tri-coloured specimens have always attracted attention, with the tri-coloured described as "watermelon", denoting the various shades of pink, white and green. It would appear that tourmaline was used early on for carvings. Dietrich3 guestimates that it's use as a decorative stone may be traced back as far as 27 BC to 395 AD. It's unusual colouration, as well as it's attractive crystal matrix configuration made it a prized collectors' item. The Dutch were aware of tourmaline as early as 1703 and utilized it's magnetic qualities to remove ash from meerschaum pipes. Dietrich relates a story dating back to 1780, where the Archduchess Marie-Anne of Austria presented a sizeable tourmaline specimen (in it's original crystallographic matrix) to the Duke Charles of Lorraine for this collection.
So, when did tourmaline become widely known as popular gemstone, with unique features? This is difficult to answer. However, one of the first proponents, George Kuntz singled out this stone early on in his career as one with interesting possibilities. Gems & Crystals: From the American Museum of Natural History4 describes George's introduction to the stone. Apparently in 1876, a young man walked into Tiffany's (where the young George Kuntz was employed) and showed Charles Tiffany a beautiful green stone with strong pleochroism. As the story goes, Kuntz was smitten by this obscure gem and fell in love. It certainly true that Mr. Kuntz did much to actively promote the use of tourmaline in jewellery and as a collector's object d'art.
However, it was during and after the Second World War, that the demand for tourmaline became significant. It was used in the production of pressure sensitive gauges for submarine instrumentation as well as other war equipment. After the Second World War, German immigration was strong and steady in the Minas Gerais State (Brazil), where some of the largest tourmaline mines were discovered. These immigrants brought with them close ties to Idar-Oberstein (a major gem fashioning centre), a familiarity with tourmaline, as well as skills in goldsmithing and jewellery fabrication. It would appear that these historical developments, i.e. the need for quartz/tourmaline crystals during the war and their subsequent discovery in the state of Minas Gerais, created a receptive environment for the introduction/production of tourmaline as a popular jewellery gemstone in North America.5
As mentioned, tourmaline's unique features make it useful as an industrial material as well as popular in jewellery. Tourmaline is a complex Aluminous Borosilicate. It is known for strong pleochroism and it's myriad of colours. Crystallizing trigonally, it has a refractive index in the 1.624-1.644 range (Uniaxial negative) and a specific averaging at 3.03. Hardness varies somewhat, but generally occupies the 7 - 7 1/2 range. Elbaite crystals are recognizable by their "prismatic form, typically elongated like pencils, with cross-sections that range between hexagonal and trigonal".6 Two-phase, elongated, irregular inclusions can be observed in red, green and blue material. Tourmaline has also been found in minerals such as apatite, fluorite, plagioclase, rutile, zircon (to name a few) as material in inclusions.
The colour of this remarkable gem is primarily the result of substitutions of transition elements for other metals. As noted, tourmaline has the largest colour representation in the gem world.
With stones spanning the entire colour spectrum from black to pink. Few generalities relating to colour apply, though pink is usually due to the presence of manganese, blue and green to ferrous oxide and iron. Chromium or Vanadium may also be present in the colour green. the formula X Y3 z6 B3 Si6 O27 (O7, OH, F) 4 has been generally accepted as encompassing all elements found in the tourmaline group. Heating and irradiation have been used to improve/create colour change, however these methods have not always produced permanent results.
Colour zoning occurs in most multi-coloured tourmaline. At one time, it was thought that the geographic location determined the precise colouration of the material. However, this is no longer a hard and fast rule. Colour zoning may be an obvious gradation as in the "watermelon" material, where the external zone is very different than the internal zone.
Dietrich7 states that a common occurrence for Elabaite crystals is "green near and including the analogus end, pink, near and including the antilogus end and early colorless through an intervening zone."
Tourmaline, as noted are strongly pleochroic and when viewed along the prism axis the colour is deeper and may appear different from that observed through the side of the crystal. Lapidaries use this information when cutting gem material and can excercise some control over the appearance. Gems cut from crystals with great depth of colour should be cut parallel to the c (prism axis), whereas those with pale coloration, may exhibit acceptable colour if cut with the table perpendicular to c.
Rare Alexandrite-like tourmaline have been found. Their daylight colouring is yellowish/brown green turning red under evening light. Chatoyancy is a feature in some tourmaline and is the result of crystals growing fluid-filled tubes to the long, prismatic axis.
OPTICAL & ELECTRICAL PROPERTIES:
Richly coloured tourmaline plane-polarizes light with an effect that is immediately visible under the microscope. Dietrich notes that a number of individuals such as Du Bois-Reymond & Schaefer (1908), Seebeck (1813), and Marx (1827) all were familiar with this quality of polarization. Marx developed tongs which had two thin slices of tourmaline cut parallel to c axis and mounted so they would polarize and reflect light. These tongs were utilized by the jewellery trade to distinguish between isotropic and anisotrophic gem material.8
Tourmaline's electrical properties have been found to be "directionally dependent of dielectric constants."9 This material possesses only singular polar axis of symmetry. Tourmaline was observed to conduct effectively across it's long axis (c). When tourmaline crystals are heated, a positive charge develops at one and a negative charge at the other. These charges reverse themselves when the crystal cools. This is referred to as a pyroelectric property. A piezoelectric charge can be developed if pressure is applied to the ends of the crystal. Schorl (black tourmaline) from Sri Lanka and Zaire show no pyroelectricity and only weak piezoelectricity.
Pyroelectricity was observed as early as 1703. We know this as records existing indicating that the Dutch referred to the stone as the aschentrekker' or ash drawer. Dietrich states10 that Jacques and Pierre Curie identified tourmaline's piezoelectric ability during studies conducted in the latter part of the 1800's. True and false pyroelectricity was noted for this material during the experiments. False pyroelectricity is actually piezoelectricity as it is caused by mechanical application of stress rather than stress due to natural forces.
Tourmaline has been used as a calibration standard for the manometer (because of it's piezoelectricity). It was also used as a standard to check possible effects of water soluble boron in mixed fertilizers.
Tourmaline is found in igneous and metamorphic rocks "as an accessory mineral but rarely of gem quality". The most common occurrence of gem material is found in granitic pegmatite dikes and is usually accompanied by topaz, kunzite, beryl, etc. The term "pegmatite dikes" was coined by the French Mineralogist, Hauy, to "describe geometric intergrowth of feldspar, quartz, and mica that petrologists today call graphic granite."11
Pegmatite is a "dikelike" body of once molten rock, which usually contains large crystals as well as rare minerals. "Gem minerals are the result of the incorporation into the pegmatite of rare elements that are unable to fit into the crystal structure of quartz, feldspar, and mica that make up the bulk of the pegmatite."12
These rare elements have been identified as beryllium, lithium, boron, manganese, phosphorus and fluorine. Pegmatites can be classified into three types - simple, zoned and complex. Simple pegmatite consists of the elements already mentioned. Zoned includes these elements and also exhibits distinct zoning (tourmaline). Complex pegmatite share all these characteristics in addition to experiencing significant alteration. This situation produces extremely large minerals, sometimes several meters in size.
Granitic pegmatites occur all over the world, according to research sources. However, only a few contain, good quality, gem-bearing material. Primary sources for tourmaline are the U.S.A. (Maine & California); the Ural Mountains; Mozambique; Nigeria and the Afghanistan. The most important sources are found in South America, specifically in the state of Minas Gerais (Brazil). Authorities such as Keller and Dietrich note that this area has yielded the greatest amount of gem-bearing material. Mining the pegmatitic belt is very difficult due to erosion and weathering of the surrounding area. Most mining activity is done by gamperios, itinerant miners who use picks, shovels and other hand-held tools. Gamperios are licensed and may work on private or publically owned land, as long as permission is given. The worker is expected to pay anywhere from 10 to 50 percent royalty to the landowner.
MAJOR PRODUCING MINES IN SOUTH & NORTH AMERICA:
Though tourmaline has been found in parts of Africa, the Ural Mountains and other European locations, however some of the most exciting finds have been established in North and South America. The largest, gem-quality tourmaline mines have been discovered in Brazil, in the State of Minas Gerais. The most significant localities are the Cruzeiro, Golconda, Virgem da Lapa and Itatiaia (Jonas) mines. The Cruzeiro mine is famous for its consistency of quality and the proliferation of superior material. Tourmalines from this area are excellent examples of complex zoned pegmatites.
The Golconda district boasts three mines, producing bi-coloured pink/green tourmalines. Mining in this area is heavily mechanized. Other gems found are Tantalite, Albite, Muscovite, Garnet and Beryl. The mines in this district are famous for producing millions of carats of find blue-green, green and rose-coloured material, the majority of which was exhumed from 1961-1967.
Virgem da Lapa is one of the most famous pegmatite sources in all of Brazil. During the 1970's yield was great, producing extraordinary tourmaline as well as aquamarine and blue topaz crystals.
Itatiaia (Jonas) mine is another significant occurrence. The Jonas mine is known for it's fine cranberry coloured material and has produced crystals of phenomenal size (several meters).13
North America has important in recent years as a influential centre of tourmaline production. Gem quality stones were discovered early in the 1800's. George F. Kuntz14 describes the geological expedition of Elijah L. Hamllin and Ezekial Holmes, in 1820 which led them to an accidental discovery of a fine, transparent tourmaline crystal at Mount Mica near, Paris, Maine. During the latter part of that century, mining operations yielded beautiful green and blue material.
Though gem material was discovered in the 1800's, it wasn't until 195915 , that new mining activity in Oxford Country (Maine) and San Diego County (California) resulted in the production of vast quantities of commercially acceptable material. In 1972, the Dutton Mine in Newry, Maine established itself as one of the largest finds on record ever and for a brief period of time, became a world-source of superior quality red and green tourmaline.
According to John Sinkankas, (author of Gemstones of North America), the quality of the material is superb who notes that the green colour is "nearly unique". The Dutton Mine in particular is famous for it's "apple-green" material, similar to the green and blue-green crystals from Southwest Africa.16 The re-opening of the Tourmaline Queen Mines at Pala, in San Diego County has yielded some exceptional pink and blue material.
Another notable find in the 1980's was Paraiba, Brazil which yielded some new and atypical offerings. Tourmalines of a medium bluish-green to medium blue-green were found as well as dark violetish-blue and purple.
The majority of fine-cut tourmaline is faceted, with the exception of chatoyant material which is cut en cabochon. Tourmaline is a Type II stone (under the North American Colour Grading System), which indicates that it is usually included.' Prevalent cuts are the table-cut, low step-cut, mixed-cut (modified brilliant) and the trap cut as well as the free-form cut. In addition to be used in jewellery and as an industrial aide, tourmaline has been carved by artists into various shapes representing nature, etc. It is not uncommon for cracks in tumbled/carved pieces to be filled in with wax or plastic to improve their appearance.
Coloured tourmaline has been fashioned into bracelets, pendants, rings, earrings and various other jewellery accessories. Of the entire array of colours, green has become extremely popular. Apart from the variety and beauty of tourmaine's green colourings, price and carat size have probably played a role in the demand for this particular colour. Equivalent size/quality Emerald is costlier and not as readily available.
In South America, where the majority of gem-quality material is still found, green tourmaline is referred to as the "Brazilian Emerald". Church leaders here wore this stone as a symbol of power and authority. The brighter, grass-green shade rivals Emerald for desirability of colour and has a higher clarity grade. The quantity of green stones which were mined in the early days of the Portuguese colonization and sent to Portugal mistakenly as Emerald will probably never be known.
Like the infamous Black Prince Ruby (which is actually a spine), coloured tourmaline has often been identified as green Beryl or coloured Corundum, among other available gem material. Dietrich notes that some famous and exceptional examples of this stone do exist however.
Catherine II of Russia was the proud owner of a 255 carat, red, grape-shaped tourmaline pendant. In recent times, stones such as the 50.59 carat, blue-green, heart shape from Mount Mica have made their way into the hands of various museums. The Governor of Maine in 1972, was presented with a multi-coloured tourmaline neck-piece, composed of stones from the Newry, Maine Mine, with the centre stone weighing 24 carats.
As mentioned, tourmaline crystals have long been prized by collectors and museums as precious and unusual examples of gem crystal configurations. Individuals such as George Kuntz, Richard Liddicoat and John Sinkankas have done much to popularize this stone.
Gemmologists and jewellers of the 1990's are better acquainted with the wide and rich assortment of tourmaline colors and have at their disposal a plethora of tests to use, when distinguishing tourmaline from other similar coloured minerals. Testing which is commonly done includes Refractive Index, use of Spectroscopy, Specific Gravity Testing as well as Microscopic Observation of features such as tourmaline's strong pleochroism.
The most recent decades have seen a greater appreciation of this remarkable stone in North America. Green, blue, red, as well as bi/tri-colour stones can be found in quality jewellery shops alongside Diamond, Ruby, Coloured-Garnet, Tanzanite and other visually stunning gems. Will green tourmaline perhaps become the rival to Emerald? Will the other colours of it's family become serious contenders with their better known rivals, for the attention of the buying public. It's too soon to say. One thing for sure, though, tourmaline will continue to thrill jewellery and gem collectors for years to come, with it's own special fascination and mystique.
1. Gubelin, Dr. Edward, C.G., F.G.A., The Color Treasury of Gemstones (Thomas Y. Crowell Company: 1975), Pg 96.
2. Dietrich. R.V., The Tourmaline Group (Van Nostrand Reinhold Company: New York: 1985), Pg.1
3. Dietrich, Pg. 191
4. Sofianides, Anna M. & Harlow, George E., Gems & Crystals from the American Museum of Natural History (Simon & Shuster: 1990), Pg. 89
5. Keller, Peter C., Ph.D., Gemstones and Their Origins (Van Nostrand Reinhold: New York: 1989), Pg. 60
6. Gubelin, Pg. 60
7. Dietrich, Pg. 122
8. Dietrich, Pg. 185
9. Dietrich, Pg. 165
10. Dietrich, Pg. 166
11. Keller, Pg. 57
12. Keller, Pg. 57
13. Keller, Pg. 66-68
14. Kuntz, George F., Gems & Precious Stones of North America (Dover Publications Inc: Second Edition: 1968), Pg 71-74
15. Hurlburt, Carl Jr., & Kammerling, R.C., Gemmology (John Wiley & Sons Inc.: New York: 1991), Pg. 263
16. Sinkankas, John, Gemstones of North America in Two Volumes - Volume II, (Van Nostrand - New York: 1976), Pg. 82- 91
This Special Issue of Gemmology Canada is published for students of the Canadian Institute of Gemmology and others interested in gemmology. The copy-right for any articles remains with the author. For further information write to the C.I.G., P.O. Box 57010, Vancouver, B.C. V5K 5G6 or phone/fax (604) 530-8569 or e-mail webmaster@cigem.ca
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