A mineral group which all the gem references I can find say has the widest variety of colors of any gemstone. The color depends on what elements have made their way into the stone's structure. When not modified by a particular color, "tourmaline" usually refers to the green varieties; these and the red ones are the most valuable. A tourmaline can have more than one color in the same stone, such as the "watermelon tourmaline." The name "tourmaline" is actually derived from a Singhalese word meaning "mixed precious stones" -- unsurprising as Sri Lanka was the first source of tourmaline.

Cat's eye tourmalines are not too rare in any color. In addition to the color zoning, tourmaline is often dichroic. Some colors have specific names such as the blue-to-teal indicolite and the red rubellite, but other names given to types of tourmaline indicate a particular mineral composition rather than a color (schorl, elbaite, and dravite). Synthetic tourmalines are also available, and other stones are sometimes mislabeled as tourmaline.

Minerology

Tourmaline is a very popular gemstone, mainly because it's available in nearly every color that a purchaser might want. Chemically it's a complex silicate of boron and aluminium which attracts a number of impurities, each of which leads to a different color. For instance, manganese impurities create a pink color (rubellite), while chromium produces green (chrome tourmaline). Red, yellow, blue, and orange tourmaline gems are all possible. Some tourmalines have two or three colors in the same crystal; "watermelon tourmaline" is green on the outer edges and transitions to clear/white and then to pink on the interior. Not surprisingly, a pure, bright color is what is most highly valued.

In addition to the gem form of tourmaline (elbaite), it also occurs as common schorl, an iron-rich form which is black and opaque, and dravite, which is translucent brown and the least common of the three. All these varieties form a hexagonal crystal and have a mineral hardness of 7 to 7.5.

Tourmaline is also useful for scientists because it changes its electrical charge when warmed or compressed, able to attract light bits of paper, lint, and ash through static electricity. Benjamin Franklin used it in his electrical experiments because of this property, and long before then, "aschentrekkers" in the Netherlands would use tourmalines to clean ash from inside pipes. Conversely, applying an electrical charge to a tourmaline crystal will cause it to vibrate.

Because of its varied colors, duplicitous gem dealers will sometimes use names such as "Brazilian sapphire" or "Brazilian emerald" to sell tourmalines. In fact, a famous tourmaline belonging to Catherine the Great was long thought to be a ruby. A lapidiary can easily distinguish a tourmaline from a more valuable gem, however, since it absorbs light along the long axis of the crystal and will appear darker from this angle than others. Gemcutters will use this property to manipulate the appearance of a set gemstone.

History

Because it is so often mistaken for other gems, tourmaline doesn't have much history or mythology of its own. It was only with the discovery of modern gem testing equipment that tourmaline can be quickly distinguished from similarly-colored ruby, sapphire, peridot, and spinel. It derives its name from the Singhalese word turmali, which means "mixed" or "mixed stones," specifically the mixtures of unidentified gem gravels found in Sri Lanka.

The Himalaya Mine was a source of pink tourmaline for the Chinese and the gem was mined heavily for the Empress Dowager Tz'u Hsi. That mine is still in intermittent use today. In the United States, Maine and California became significant sources of tourmaline beginning in the 1820s and 1870s, respectively. Native Americans in California discovered and used tourmalines long before that, however. Today, Brazil is the world's top producer of gem-quality tourmalines.

Mythology

Tourmaline is sometimes known as the "peace stone", capable of inducing calm and dispelling fear in its wearer. It is also said to encourage creativity and artistic intuition because its varied colors can express every mood.

Today

Tourmaline's variety of colors and relative affordability makes it popular among mineral collectors and gem lovers alike. It is mined primarily in Brazil, California, Maine, Sri Lanka, Italy and parts of Africa, Afghanistan, and Pakistan. Each region is generally known for producing certain colors -- pinks in California, greens in Maine -- although Brazil is known for producing almost every color possible, including vivid blues and greens that had never been seen before 1989.

Tourmaline is the birthstone for the month of October, together with opal, and is the symbolic gemstone for the eighth wedding anniversary.

"The chemistry of it is more like a medieval doctor's prescription than the making of a respectable mineral"

-- John Ruskin (1819-1900)

Ruskin wasn't far off. Tourmaline has a simplified chemical formula of:

XY3Z6(BO3)3Si6O18(O,OH,F)4

Tourmaline, although widely considered to be one mineral, is best described as a solid solution series. The cation sites X, Y and Z in the formula above may be occupied by a wide variety of metal ions. The most common substitutions are:

X: Na+, Ca2+, K+
Y: Al2+, Mg2+, Fe2+, Fe3+, Mn3+, Li+, V2+
Z: Al2+, Mg2+, Fe3+, Mn3+, Cr3+
Now that that bit of ugliness is out of the way, we can take a look at some of the common end members in the tourmaline system. (Please note that the following formulae are ideal end members. As with most minerals that occur in solid solution, the degree of cation replacement will vary throughout the minerals in a rock formation, and even within the crystals themselves.)

Elbaite: Na(Li,Al)3Al6Si6O18(BO3)3(OH)4
Schorl: NaFe3(Al,Fe)6Si6O18(BO3)3(OH)4
Dravite: NaMg3(Al,Fe)6Si6O18(BO3)3(OH)4
Uvite: Ca(Mg,Fe)3Al5MgSi6O18(BO3)3(OH)3

Structure:
Tourmaline belongs to the group of minerals known as cyclosilicates or ring silicates. A tourmaline molecule consists of two overlapping rings. One ring is comprised by six silica tetrahedra, and overtop (or underneath, depending on your perspective) of this lies a second, more complex ring. This second ring has two parts of its own: an inner "core section" (the three Y cations and the anions from the O, OH, F group) and an outer section (the Z cations, which bind to the oxygen atoms in the silica tetrahedra, the X cations and the borate groups).

Occurance:
Tourmaline-group minerals are relatively common. They occur as accessory minerals in many igneous and metamorphic rock suites. The most striking examples of tourmaline crystals, however, are found in pegmatites and greisen, and can often reach several inches in length.


Sources:
Deer, W.A., Howie, R.A. and Zussman, J., The Rock Forming Minerals
Simon & Schuster's Guide to Rocks and Minerals
Amethyst Galleries - http://mineral.galleries.com/minerals/silicate/tourmali.htm

Tour"ma*line (?), n. [F. tourmaline, cf. It. turmalina, tormalina, NL. turmalina, turmalinus; all fr. tournamal, a name given to this stone in Ceylon.] Min.

A mineral occurring usually in three-sided or six-sided prisms terminated by rhombohedral or scalenohedral planes. Black tourmaline (schorl) is the most common variety, but there are also other varieties, as the blue (indicolite), red (rubellite), also green, brown, and white. The red and green varieties when transparent are valued as jewels.

[Written also turmaline .]

Crystals of tourmaline when heated exhibit electric polarity (see Pyroelectric, n.). Tourmaline is also used in the form of a polariscope called tourmaline tongs.

 

© Webster 1913.

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