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Tourmaline

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Tourmaline is the traditional birthstone for the month of October. 

Tourmaline is the name given to a family of borosilicate minerals of the cyclosilicate superclass. Members are of complex and variable composition, but all members have the same basic crystal structure. There are 11 species in the Tourmaline group, including elbaite, dravite, schorl (black), and liddicoatite. numerous varieties are also recognized, including indicolite (blue), achroite (colorless), rubellite (pink or red), and verdelite (green), and these variety names can apply to more than one tourmaline species. Elbaite forms a solid-solution series with dravite, and dravite forms a solid solution series with elbaite and schorl. 

Formula: A(D3)G6(Si6O18)(BO3)3X3Z
A = Ca, Na, K, Pb or is vacant (large cations);

D = Al, Fe2+, Fe3+, Li, Mg2+, Mn2+, Ti (intermediate to small cations - in valence balancing combinations when the A site is vacant);

G = Al, Cr3+, Fe3+, V3+ (small cations);

Si can sometimes have minor Al and/or B3+ substitution.

X = O and/or OH;

Z = F, O and/or OH.

Note: In the formulas of the group members, Mindat has put the D site cations in parentheses in order to facilitate the assignment of the different cations to the crystallographic sites. - Additional cations are reported substituting in minor amounts for the principal cations of the G and D sites. - The X and Z sites are usually combined in the simplified formula for the Group as Z4.

Crystals of tourmaline are generally prismatic. Colored crystals are very strongly dichroic and frequently display color zoning. Tourmaline is abundant and is best-formed crystals are usually found in pegmatites and in metamorphosed limestones in contact with granitic magmas. Tourmaline minerals are resistant to weathering, so they accumulate in gravel deposits--for the same reason, tourmaline is an accessory mineral in some sedimentary rocks. The origin of its name is the Singhalese word turamali, meaning "gem pebbles". Reported by Christianus-Fridericus Garmann in 1707. The name "tourmali" was a generic name used in Ceylon [Sri Lanka] for colored gems, mostly zircons. About 1703, it had been discovered by Dutch lapidaries that some of the "zircons" arriving in the Netherlands were actually a previously undescribed mineral. Several names were given to the new mineral including "Pierre de Ceylan, by Lemery in 1717. Tourmalin, as a more or less specific mineral name, was used by Rinmann in 1766. Hill called it Tourmaline Garnet in 1771 and Richard Kirwan shortened the name to "Tourmaline" in 1794.

Tourmaline's piezoelectric properties mean that it is also an important industrial material. It is employed in pressure devices such as depth-sounding equipment and other apparatus that detect and measure variations in pressure. It is also used in optical devices for polarizing light.

Gem-quality tourmaline occurs in numerous localities. Although many of its transparent varieties are valued as gems, most tourmaline is dark, opaque, and not particularly attractive except as well-formed mineral specimens.

 

Probably the most common tourmaline is schorl, a black, opaque, iron-rich mineral. Its prismatic crystals may reach several yards long. Dravite is a very dark-colored (usually brown) tourmaline, rich in magnesium. It is sometimes cut as a gem and its color can be lightened by heat-treatment. It is found in the gem gravels of Sri Lanka, and in the US, Canada, Mexico, Brazil, and Australia. 

 

 

 

The most dramatic of the color-zoned gem tourmaline is the "watermelon" tourmaline, which, when sliced across the crystal, shows a red or pink center surrounded by a green rim. Some crystals are pink at one end and green at the other. Color change, from deep-green to dark-red, with an increasing path length of light, occurring in Cr-bearing tourmalines, is known as the Usambara effect.


Elbaite provides the most gemstone material. Usually green (verdelite), yellow-green is the most common of gem tourmaline varieties, but it also can be pink-red, blue, and colorless. Emerald green is the rarest and most valuable, and until the 18th century, it was often confused with emerald. Most emerald-green tourmaline comes from Brazil, Tanzania, and Namibia. People have probably used tourmaline as a gem for centuries, but until the development of modern mineralogy, they identified it as some other stone (ruby, sapphire, emerald, and so forth) based on its coloring.

                                    

One of the earliest reports of tourmaline in California was in 1892. In the late 1800s, tourmaline became known as an American gem through the efforts of Tiffany gemologist George F. Kunz. He wrote about the tourmaline deposits of Maine and California and praised the stones they produced.

In spite of its American roots, tourmaline’s biggest market at the time was in China. Much of the pink and red tourmaline from San Diego County in California was shipped to China because the Chinese Dowager Empress Tz'u Hsi was especially fond of the color. There, craftsmen carved the tourmaline into snuff bottles and other pieces to be set in jewelry. San Diego County's famed tourmaline mines include the Tourmaline Queen, Tourmaline King, Stewart, Pala Chief, and Himalaya. The miners became so dependent on Chinese trade that when the Chinese government collapsed in 1912, the US tourmaline trade also collapsed. The Himalaya mine stopped producing large volumes of gemstones. Other mines in San Diego County, like the Stewart Lithia mine at Pala, still produce sporadic supplies of gem-quality tourmaline.

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