Formula: AD3G6 (T6O18)(BO3)3X3Z
Where:  A= Ca, Na, K, Pb or is vacant; D = Al, Fe2+, Fe3+, Li, Mg2+, Mn2+, Ti;  G = Al, Cr3+, Fe3+, V3+; T = Si almost exclusively, X = O and/or OH; Z = F, O and/or OH

Species: Silicates – (Cyclosilites Borosilicates)

Hardness: 7

Specific Gravity: 2.9 – 3.1

Crystal System: Trigonal

Name: Reported by Christianus-Fridericus Garmann in 1707. The name “tourmali” was a generic name used in Ceylon [Sri Lanka] for colored gems, mostly zircons. Hill called it Tourmaline Garnet in 1771 and Richard Kirwan shortened the name to “Tourmaline” in 1794.

Tourmaline (synonymous with tourmaline group) comprises a family of borosilicates minerals 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, and liddicoatite. Numerous varieties are also recognized including indicolite (blue), achroite (colourless), rubellite (pink or red), and verdelite (green), and these variety names can apply to more than one tourmaline species. trigonal borosilicates of the cyclosilicate superclass.

Elbaite provides the most gemstone material. Usually green, it can have many colors, including pink or red, blue, and green; it can also be colorless. Yellow-green is the most common of all gem tourmaline color varieties. Emerald-green is much rarer and more valuable, and, until the 18th century, it was often confused with emerald. The most dramatic of the color-zoned gems is the “watermelon” tourmaline, which, when sliced across the crystal, shows a red or pink center surrounded by a rim of green. Some crystals are pink at one end and green at the other end.

Crystals of tourmaline are generally prismatic. Colored crystals are very strongly dichroic and frequently display color-zoning. Tourmaline is abundant, and its best-formed crystals are usually found in pegmatites and in metamorphosed limestones in contact with granitic magma. Tourmaline crystals are resistant to weathering, so they accumulate in gravel deposits, the origin of its name is the Singhalese word turamali, meaning “gem pebbles”. For the same reason, tourmaline is an accessory mineral in some sedimentary rocks. Gem-quality tourmaline occurs in numerous localities. Tourmaline’s piezoelectric properties make it an important industrial mineral. 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.

Although many of the transparent varieties are valued as gems, most tourmaline is dark, opaque, and not particularly attractive except as well-formed specimens. Probably the most common tourmaline is schorl, a black, opaques, iron-rich mineral. Its prismatic crystals may reach several meters long. Dravite is a very dark-colored (usually brown) tourmaline, rich in magnesium