Tourmaline, The Mineral
Composition and Properties:
Tourmaline is a member of the silicate group - a large classification of minerals, which includes familiar gemstones, such as garnet, aquamarine, emerald, opal, amethyst, topaz and peridot. It is most closely related to quartz and opal. The name Tourmaline (from the Singhalese turmali or "mixed") represents a subgroup of minerals in this group with similar physical characteristics and chemical properties. All tourmalines share the same crystal form and molecular structure.
Chemically, tourmaline is one of the most complex of all minerals, which accounts for its extraordinary range of colors. Different varieties occur in black, clear, green, pink, red, blue, yellow, brown, orange, purple and white, sometimes with two or more colors sharing the same crystal. The presence or absence of certain elements in the chemical formula (the elements in parenthesis in the formula above) are what create these variations.
Tourmaline crystals are hexoganal (six-sided) and usually elongated (longer than they are wide), striated (marked by many thin, parallel grooves) and prismatic (sides are similar in length and width). The typical shape can best be described as pencil-shaped crystals of varying length.
Tourmaline has a vitreous (glassy) luster, and a hardness of between 7 and 7.5 on the Moh's scale, making it slightly more resistant to scratches than Amethyst and Citrine, but a little more scratch-prone than Topaz. Tourmaline's average specific gravity is 3.15, making it moderate in weight for its size.
Varieties of Tourmaline:
The most common types of tourmaline are:
- Elbaite - Nearly all tourmaline gemstones are elbaite. Names often used for sub-types include:
- Rubellite - Red to dark pink
- Indicolite - Blue
- Achroite - Clear, colorless
- Siberite - A name sometimes used for the purple variety
- Paraiba - Rare electric blue/green tourmaline found in Paraiba, Brazil and Nigeria.
- Watermelon Tourmaline - Pink and green in the same crystal. Usually used to describe crystals where one color is in the center of the crystal with the other color surrounding it.
- Schorl - Schorl is a black variety of tourmaline. It is not used in modern-day jewelry, though in the past it was used for mourning jewelry.
- Dravite - A brownish tourmaline variety, not used in jewelry.
Less common varieties include Liddacoatite, Uvite, Chromdravite and Buergerite. These are very uncommon and specimens are highly prized by mineral collectors
Where Tourmaline is Found:
Tourmaline is found throughout the world. Significant sources for the mining of gem quality tourmaline are Brazil, Africa ( Madagascar, Namibia, Nigeria, Afghanistan and Tanzania), Pakistan and Sri Lanka. The United States (particularly southern California and Maine), Italy, Myanmar, Nepal and the USSR are also notable sources of gem deposits. Certain varieties, including Schorl and Dravite, are commonly found in Germany, Austria and Slovenia.
All colored tourmaline is plechroic, meaning its color changes depending upon the angle from which it is viewed. Skillful gemcutters take this into account to bring out the best color in a particular stone. Some tourmaline also fluoresces yellow under light from a short-wave ultra-violet lamp.
Probably the most unusual qualities of tourmaline are its electrical properties. Due to its atomic structure, tourmaline can generate an electric charge when heated (pyroelectricity) or when subjected to friction (piezoelectricity). These characteristics were first mentioned in an anonymous document published in 1707 and were a subject of fascination among notable scientists of the 18th century, including France's M. Louis Lemery, German physicist Franz Aepinus, and even Benjamin Franklin! Carl Linnaeus, the famous Swedish botanist and the father of modern taxonomy, dubbed tourmaline "The Electrical Stone."
Because of its electrical nature, tourmaline was once used to clean the ashes from pipes (hence the Dutch nickname, Aschentrekker.) It is still used today in piezoelectric sensors.