Gemmology Canada - Special Issue
by Marianne Odber, A.G. (C.I.G.), Surrey, B.C.
I never knew that Tourmaline was such an interesting gem mineral. I guess that shouldnt be a surprise considering that they can be found in some of the most vibrant colors, as well as every color of the rainbow. It is a family of related minerals, all having essentially the same crystal structure but differing in chemical composition, color, and other physical and optical properties.
Tourmalines are formed in crystalline shchists, in granites, granite pegmatites, marbles and other metamorphic rocks. It can be found as inclusions in Quartz, Beryl, Zircon and is sometimes found in Feldspar. Tourmaline pegmatite is formed as a course grained granite by slow cooling of magma at a considerable depth in the Earths crust. These rocks whose slower rate of cooling in the middle and lower parts of the crust, made it possible for quite large crystals to form from the original molten residues. As the temperature of the magma began to drop, minerals separated out in a process known a fractional crystallization. Tourmaline was not one of the first to solidify but as the magma continued to cool, Tourmaline crystals began to form. Tourmaline occurs in pegmatites with other minerals such as Lepidolite, Microline and Spodumene. It has been found with Andalusite, Biotite, Siderite, Quartz, Molybdenite and Cassiterite in other types of mineral formations. Deposits can also be found as alluvial deposits after the minerals have been washed down from their original sights by rain, wind and sometimes fast moving rivers.
There are nine distinct mineral species in the group as well as a
variety of names that have been given to specific color varieties
found worldwide. These are by name, Elbaite, Schorl, Buergerite,
Dravite, Chromdravite, Uvite, Ferridravite, Tsilaisite, and
Liddiocoatite, named for a US gemmologist. They come in all colors
including colorless and black. Crystals are frequently color zoned
along their length, forming bicolor, tricolor, and multi- color
crystals. Some of the multicolored crystals can contain up to fifteen
different colors or shades of the same color.1
Some crystals are concentrically zoned, for example the popular
variety known as watermelon tourmaline which appears with a pink
center, a white ring around that with green around the outside of the
crystal. Dravite and Buergerite are usually black or brown and
sometimes colorless where as Uvite is black or dark brown or dark
green in color. Verdelite is the name given to a variety of
Tourmaline that is a rich green color which resembles Emerald, and
Chromdravite is a species that has an intense green color as well.
Some other color varieties that have widely used names are known as
Rubelite, Indicolite and the colorless variety Achroite. These
Tourmalines are the more desirable varieties because they are found
in colors ranging from the pink and red shades of Rubelite , the
electric blue of the Indicolite and the intense green of
Chromdravite. Schorl, was used in Europe during the Victorian Era as
mourning jewelry. It is black, blue or blue-green in color, very
common and has little value if any in the gem market today.
Tsilaisite is a yellow - brown variety and Ferridravite is dark brown
to dark olive-green in color.
Elbaite occurs in a large range of colors or shades and Liddicoatite is similar to Elbaite with some minor differences in chemical composition. The main chemical components of Tourmaline are silica and alumina in nearly equal proportions. These make up about three-fourths of the whole. The remainder consists of boracic acid, ferrous oxide, manganese oxide, magnesia, lime, soda, potash, and lithia, which are not all present in any one specimen.
Tourmaline is about a seven to seven and a half on the hardness scale and has no cleavage. It doesnt tend to break or chip easily and can be worn as jewelry quite safely.2
When it does fracture it is conchoidal in appearance, shell like with a smooth curved surface. The sheen on the fractured surface is greasy but on the crystal faces the luster tends to be vitreous. Tourmaline is anisotropic but sometimes shows only one shadow edge on the refractometer causing it to appear as isotropic. It has a refractive index of about 1.62 -1.65 with a birefringence of 0.014 - 0.032. Sometimes when examining faceted material with magnification, doubling of the back facet edges can be observed due to the high double refraction. It is strongly pleochroic and because of this, dark stones must be cut so that the table lies parallel to the main axis. In pale stones, the table should be perpendicular to the main axis in order to obtain the deeper color, but the colorless variety known as Achroite can be cut in any direction. Since it is colorless it does not display the strong dichroism that the other color varieties do. The dark green and brown tourmalines have the strongest dichroism. Uni-colored tourmaline is quite rare.
Tourmaline has a weak spectrum and is not recomended for use as diagnostic proof. How ever, the blue and green stones have a strong band in the green at 498 nm and a weak one in the blue at 468 nm. Pink and red stones have a broad absorption band in the green and two narrow bands in the blue at 450 nm and 458 nm.3
Tourmaline has two interesting properties known as pyro-electricity and piezo- electricity. When tourmaline is heated and then cooled, or when it is rubbed, it acquires an electric charge. The Dutch used this crystal material to pull ash from their Meerschaum pipes because particles of dust and ash would stick to it, consequently they named this gem material Aschentrekker, ( ash puller ). For this reason, Tourmaline has to be cleaned more often than other gemstones.4
Tourmaline belongs to the trigonal crystal system and is usually found as prismatic crystals with rounded triangular cross sections. It is vertically striated, parallel to the c axis. Sometimes wavy fracture lines are present perpendicular to the c axis.
Inside a Tourmaline crystal are a number of different mineral inclusions that vary in shape as well as content. There are elongated or threadlike cavities, Mica, Apatite or Zircon crystals and sometimes hornblende inclusions. Some specimens have two phase inclusions consisting of a liquid and a gas bubble. Inclusions run parallel to the length of the crystals and when densely packed may produce a chatoyant effect that yields cats- eye gems when fashioned as cabochons. In red Tourmalines there are sometimes filled fractures that are used to improve the clarity of the stone. There may also be flat films that reflect light and appear dark.
Tourmaline has a number of factors which affect its value in the market today. Due to the low dispersion that it has, the color is extremely important for gem material to be used in jewelry. The more intense the color of the gem, the more valuable it is. Clarity is also a large determining factor, if the stone has a very clear appearance the stone is obviously more valuable. It is good to remember that rarity of the specimen can also determine its worth.
The yellow- green tourmaline is the most common of the color varieties. The green variety, when it contains chromium is much more valuable than the green gem that does not. This can bring the value of the stone up to $1000 per carat wholesale from the price of about $20 per carat for very low, quality up to $400 per carat. You can check for the presence of chromium by using a Gemmology testing filter known as the Chelsea Filter. By viewing the stones through this filter, the ones that contain chromium will appear a reddish or pinkish color. The other green stones are inert and will appear green through the filter.
The yellow, orange - brown or golden varieties of Tourmaline sell for about $30 to $350 per carat retail. The red variety named Rubelite remains red in both incandescent and daylight. This variety is more valuable than other red Tourmalines because most of the red Tourmaline usually turns brown in incandescent lighting. Most of these red Tourmalines are undesirable, therefore less valuable. Unfortunately a higher clarity is usually found in the red to brown- red Tourmalines than in the true red Rubelite variety. A high clarity true red Rubelite can bring up to $2000 per carat wholesale. Sometimes the Rubelite variety is filled with fillers to improve its clarity, but it still has a lesser clarity after treatment than the red - brown variety. Another variety that is valued high and desired by collectors is the variety known as Indicolite. It displays a deep, sky blue with strong color intensity, and it sells for about $100 to $800 per carat retail. There is a variety of this type of Tourmaline which was found in Paraiba Brazil which is the rarest and most valuable. Some of the better quality stones have sold for up to $10,000 per carat wholesale.5 A Paraiba Tourmaline with an intense green or violet color is rare and highly desirable for collectors. Unfortunately these gems will get more difficult to find and very costly to own because the Paraiba Tourmaline has been mined out. The bi- colored and multi- colored varieties are the most valuable when they have distinct intense colors with sharp boundaries and no fractures.
Cats-eye Tourmaline which comes in pink and green is more common,
therefore less valuable than the red or blue color. Retail prices for
these chatoyant varieties range from $30 to $700 per carat.
A variety of Tourmaline known as color change Tourmaline is rare and valuable because it is considered a collectors item. It appears yellowish to brownish green in daylight and orange red under incandescent light.
Lesser quality gems are sometimes treated or enhanced to appear as better quality and to possibly command a better price. To improve its clarity, the Rubelite variety is treated with fillers but it still remains a lesser quality gem. Heat treatment is used on Tourmalines of lesser color to intensify the existing color especially on Indicolite, pink and red varieties. It is used on light yellow or green stones to produce orange or golden Tourmaline. Heat is also used on light pink to produce colorless stones. Stones are sometimes treated with gamma irradiation to enhance the color, but the color is not permanent and may fade when exposed to extreme light, or the stone may fracture when exposed to extreme heat.
Cats-eye tourmalines have been treated with epoxy resins to improve transparency and to seal the tubes creating the cats eye. These fillers also prevent dirt from entering the tubes.
Tourmaline is synthesized for research purposes only but is commonly simulated by using synthetic Spinel.
The most important Tourmaline supplier is Brazil, where the blue variety known as Indicolite can be found. It is also the origin of the famous Paraiba Tourmalines that are widely sought by collectors. Other deposits can be found in Afghanistan, Australia, Burma, India, Madagascar, Malawi, Mozambique, the United States and Zaire. Some deposits have also been found in Europe in places such as Italy and Switzerland.6
The world is rich in Tourmaline and for now the availability is such that everyone can have one for an affordable price. This may not be so true for the future, for as the demand increases, the cost will surely follow.
1 Gemstones, Eye
Witness Books 1994 Cally Hall Pg. 103
2 Arem Gems And Jewelry 1977, Pg. 81
3 Gemmology 1991 Peter G. Read Pg. 326
4 Gemstones of the World Schumann page 112
5 GemstoneBuying Guide 1998 Renee Newman Pg. 130
6 Arem Gems And
Jewelry 1977, Pg. 81
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 email@example.com <
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