RubellitePink Tourmaline, Pala    TOURMALINE     Particolored Tourmaline
(ACHROITE, RUBELLITE, INDICOLITE, VERDELITE, WATERMELON)

   Tourmaline, from the singhalese tourmali, meaning mixed stone, is an extremely complex group of hydrous minerals containing Li, Al, B, and Si, plus varying quantities of alkalis (K, Na) and metals (Fe, Mg, Mn). It is the principle boron-containing mineral in the crust and has its genesis in both igneous (principally pegmatites) and metamorphic rocks. Tourmaline classification, even to the dedicated mineralogist, represent a confounding array of names (Elbaite, Liddicoatite, Tsilaisite, Dravite, Uvite, Schorl, etc.) that have their basis in slight variations in chemistry and crystalline structure. Fortunately for gemologists (and gemology students!), most attractive gem material falls under the group of tourmalines known as Elbaite [Na(Li, Al)3Al6(BO3)3Si6O18(OH)4]. Gem varietal names, based on color, for this group are: Achroite (colorless); Rubellite (pink to red); Indicolite (various shades of blue); Verdelite (various shades of green); Watermelon (pink center with outer rim of green: opposite is called Reverse Watermelon). The adjectives bi-color or parti-colored describe single stones or crystals that show two (bi-) or more (parti-) colors that are not concentrically arranged. Chrome tourmaline is a name of fairly recent vintage used for African (Namibia, Tanzania) material that has an outstanding, bright, vivid green color, resembling fine emerald. As the name suggests, the chromophore in this variety is principally Cr, but also probably involves vanadium. The source of color in other varieties is less clear and is probably complex; not all stones of a given color contain the same chromophores. In general, most colors can be attributed to Fe of valence +2, +3 or both, and/or Mn. Finally, several authors suggest dropping the somewhat antiquated, color-specific names in favor of simple color adjectives, e.g. green tourmaline, blue tourmaline. I agree; the names given above should serve for reference only. Don't memorize them.

    Nearly all gem tourmaline is found in pegmatites. The principle commercial source is Brazil. Important other sources are the U.S. (Maine and S. California), U.S.S.R., Sri Lanka, Burma, and Madagascar, Tanzania, and Afghanistan.


Properties

  • Crystal System: Hexagonal (trigonal)
  • Habit: As well-formed, elongate, trigonal prisms, with smaller, second order prism faces on the corners. Prism faces are often striated parallel to direction of elongation (c axis). The rounded triangular cross-sectional shape of tourmaline crystals is diagnostic; no other gem mineral has such a shape.
  • Hardness: 7-7.5
  • Cleavage: none
  • Fracture: conchoidal; sometimes forming globular or spherical lumps ("nodules") of inclusion-free material.
  • Toughness: good
  • Specific Gravity: 3.0-3.12; varies with color:
    • pink - 3.03
    • red - 3.05
    • pale green - 3.05
    • brown - 3.04-3.10 (3.06)
    • dark green - 3.08-3.11 (3.08)
    • blue - 3.05-3.11 (3.10)
    • yellow-orange - 3.10
    • black- 3.08-3.20 (3.15)
  • R.I.: commonly 1.62-1.64; large reported range from 1.610-1.675.
  • Birefringence: high; average about 0.018
  • Dispersion: Medium; 0.017
  • Pleochroism: Strong, diagnostic, visible to the naked eye; varies with color but most varieties exhibit strong (some very strong) absorption parallel to the c axis (i.e. look darker), and less absorption perpendicular to c. Dramatic dichroic color change noticeable in some rubellite (pink to orange) and other varieties.

Distinguishing Properties

  • High birefringence
  • Strong Dichroism
  • Habit for rough material
  • Any transparent gem having a mean R.I. of 1.63 and a birefringence of 0.015-0.020 is tourmaline.

Sources

    Tourmaline is widespread in metamorphic, igneous and sedimentary rocks. Gem Elbaite is, however, nearly restricted to pegmatites. Literally thousands of tourmaline-bearing pegmatites are known; only a few hundred apparently contain gem quality material in mineable quantities.

  • Brazil
    • Largest producer of gem tourmaline. All colors. Most mines in the state of Minas Gerias; the most prolific and famous pegmatite mines are Cruzeiro, Galconda, Virgem da Lapa and Jonas (Itatiaia).
    • Some of the finest red ("Cranberry") tourmaline was discovered at the Jonas mine in 1978.
    • Recently discovered deposit in the state of Paraiba has yielded unusually vivid purples, blues and greens. Chromophore in some of these is copper, and most contain a fairly high gold content. These are some of  the most expensive tourmalines on the market, commanding prices that were previously unheard of for tourmaline.
  • Sri Lanka
    • Second most prolific production. From the same gem gravels that produce corundum and beryl. All colors; pinks rare.
  • U.S., Southern California
    • Pala District, San Diego County.
    • Bicolored crystals common; fine pinks.
    • Mined extensively from 1900-1912; pinks and reds favored by Chinese for carvings and other ornamental uses. Market all but evaporated with the collapse of the Manchu dynasty in 1912.
    • Some mines have recently reopened and are being worked on a fairly large scale.
  • U.S., Maine
    • Near Newry (1970's) and Paris (since mid 1800's)
    • Newry deposit noted for watermelon, red and apple-green crystals; blues usually too dark to cut. Some very large crystals ("tourmaline logs") discovered as well; highly prized as mineral specimens.
  • Tanzania
    • Notable for recently discovered, exceptional emerald green stones ("Chrome tourmaline"). These make truly exquisite gems, rivaling the finest emeralds in color.
  • Others - Kenya, Zambia, Afghanistan, Namibia (pink, purple, watermelon), Nigeria (bicolors), Mozambique, Madagascar.

Shaping and treatment

  • Color zoning, inclusions, dichroism need attention. Rectangular step-cut stones cut with the table parallel to c-axis in deeply colored stones often have steep pavilion facets at the ends of the rectangle to minimize the deep color parallel to c.
  • Cats eye and star stones are known; chatoyancy due to oriented, tubular, fluid inclusions.
  • Heat treatment and irradiation have highly unpredictable affects; very different results for same colored stones from different localities. Site-specific heat treating is apparently widely used, however.
  • Heating used to lighten blue and green stones, and also to make some brownish stones pink. Irradiation sometimes used to darken stones, e.g. to darken light pink. Both heating and irradiation tends to make stones more brittle, thus usually treated after cutting.
  • Red tourmaline produced by irradiating pinks. Bicolored red and green have also been produced this way.

Pricing and Valuation (wholesale)

  • Not a particularly rare gem. Most highly prized are pinks, reds, and blues. Less expensive are greens, oranges, and browns.
  • Depth of color and tone important; most inexpensive gems are too dark or light and contain undesirable overtones (e.g. blue with gray, pink with brown).
  • Clarity is a factor; most tourmaline is lightly to heavily included as rough. Well cut stone will not show noticeable inclusions (hidden near girdle).
  • Blue - finest have color of fine aqua., quite rare. Most material too dark or has gray overtone. Tend to be less included than pinks or reds. Good stones: $100-$300/carat in ten carat sizes. Paraiba blues have sold for as much as $10,000/ct!
  • Red, Pink - usually quite included. Bright red to purplish red, very clean $400-$700/carat for 10 carat stones. Less for smaller. "Average" pink stones were selling in 1989 for about $10-$125/carat in the 1-2 carat range.
  • Green - commonest color; don't accept too dark a stone. Most expensive are rare, yellowish green, like emerald. Other greens - much less than $100/carat. Excellent chrome tourmaline was selling for about $275/carat in the l-1.99 carat range in 1989. 3 or more carat stones were $450-$800/carat.
  • Bicolor - good, clean, pink/green bicolored stones of 2 or more carats sell for about $100-$300/ct.

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1998
Updated 08/20/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin