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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